Why does the new Ordinariate Mass consist of a combination of BCP, traditional Roman Rite, and modern Roman Rite elements? I commented on Debra Gyapong's blog on all this, but the comment became as lengthy as a posting, so here is my essay...
At random (well, not really), I plucked from my bookshelf a 1952 copy of The Priest's Book of Private Devotion, a classic Anglo-Catholic manual first issued in the late 19th C., and turned to "Directions and Devotions for the Celebrant" (at the Holy Communion). This provides the following items:
1. The Prayers at the Foot of the Altar, incl. the Confiteor - all this (except for the final collect Aufer a nobis) is now part of the Ordinariate Mass;
2. The 1662 BCP Communion service, from the start to the Gospel - the Ordinariate Mass omits the prefatory Lord's Prayer, gives the Decalogue and the Summary of the Law (a happy innovation of the Nonjurors) as options, inserts here the Kyrie and Gloria in excelsis (as the 1549 BCP appointed), puts the Dominus vobiscum before the Collect (omitting the 1662 Collect for the Queen), and uses the readings appointed in the modern Roman Lectionary according to the Revised Standard Version;
3. Short prayers before and after reading the Gospel - I have no information on this, but I imagine the Ordinariate Mass appoints such;
3. The 1662, from the Creed to the Offertory sentence;
4. The Tridentine offertory prayers (none for the censing, since this is all for Low Mass), excluding only the Orate fratres, Suscipe sancta Trinitas and Secret - the new Ordinariate Mass has all of these as "Form I" of its offertory;
5. The 1662 Prayer for the Church Militant - the Ordinariate Mass puts this, the intercessions, before the offertory, and has various forms for this, including (I believe) a suitably supplemented form of this prayer, including prayer for the departed and commemoration of the saints;
6. The 1662 'Penitential Act' ("Ye who do truly" to the Comfortable Words) - the Ordinariate Mass has all of this, with some slight modifications to the text (omitting "and take this holy Sacrament to your comfort" and putting the 'absolution' in the first person plural), but again putting it all before the offertory;
7. The 1662 Sursum corda, Preface, Sanctus and Benedictus (the last not being 1662) - again, this is all taken into the Ordinariate Mass;
8. The 1662 Prayer of Humble Access - put immediately before the reception of Holy Communion in the Ordinariate Mass;
9. The 1662 Prayer of Consecration, supplemented with the 1549 Anamnesis and the 1549/1662 Prayer of Oblation - all this is replaced in the Ordinariate Mass with the Roman Canon (which in The Priest's Book of Private Devotion is given in full, in Latin, including also all the prayers from the Lord's Prayer to the Communion prayers, directly after these "Directions and Devotions" - suggesting that some at least simply turned to it and used all those prayers instead);
10. While in the 1662 the Communion followed directly after the Consecration, these "Directions and Devotions" instead give, in English, all the prayers in the Roman Mass after the Canon, that is, "Let us pray. As our Saviour... Our Father... Deliver us, O Lord... The peace of the Lord... This commingling... O Lamb of God...", the three prayers before Communion, all the formulæ and psalm-verses of the priest's communion, and the two prayers at the ablutions - the Ordinariate Mass seems to have most of these (the Lord's Prayer, the Libera nos in its immemorial form, the prayer Domine Jesu Christe, qui dixisti and the Pax, the Agnus Dei, Ecce Agnus Dei and Domine non sum dignus, threefold) but also, transferred from above, the Prayer of Humble Access, and a special versicle at the Fraction, quoting the Apostle, but whose usage here is derived ultimately from 1549;
11. The 1662 resumes with the Lord's Prayer after Communion (a repetition wisely omitted in the Ordinariate Mass), the Prayer of Thanksgiving (included in the Ordinariate Mass, with some minor corrections to avoid the appearance of receptionism), the Gloria in excelsis (moved back to its Roman Rite position in the Ordinariate Mass) and the blessing (included in the Ordinariate Mass).
12. Then comes the Last Gospel - also included in the Ordinariate Mass.
So, as evidently various Anglo-Catholics in 1952 took up the 1662 BCP, and prefaced it with the Prayers at the Foot of the Altar, interlarded it with the Tridentine Offertory and Communion prayers, and tacked on the Last Gospel at the end, the ritual reason why the Ordinariate Mass now contains most of these features is easily understood.
I have seen, also, various Anglican Missals – none of them "official" publications, but used in Anglo-Catholic parishes all over the world; most were old, but I've seen modern reprints on sale at the Anglican bookshop next to St Patrick's Cathedral in Melbourne – which similarly consist of greater or lesser combinations of the BCP and Missale Romanum. My former parish priest, himself once an Anglican minister (and now a Catholic bishop; "I've been ordained five times!" he joked) once showed me his old English Missal, which had three Eucharistic Prayers: the BCP Prayer of Consecration and Prayer of Oblation back to back (the "interim rite"), the Roman Canon in English, and the Roman Canon in Latin.
Now that I think of it, I have a copy of The English Missal for the Laity (Knott, 1933), which the average Catholic would probably mistake for a hand-missal to bring along to Latin Mass; it certainly contains enough parallel Latin-English! Looking to the texts provided for Advent Sunday, a Catholic would be puzzled as to why the Collect, Epistle and Gospel wouldn't match those at Mass (if they noticed), and perhaps suspicious as to why the third collect is for the "chief bishop", not Pope. (That said, among the Votive Masses are those for the Election of the Pope, and on the Day of his Election and Coronation.) As well, the Sundays after Pentecost are numbered after Trinity instead...
Looking to the Knott Missal's Ordinary of the Mass, all would seem alright, but why does the Lord's Prayer, in its Protestant wording, and a Collect for the grace of the Holy Spirit, separated out by heavy lines, appear a little before the Introit? Could it be meant as a private devotion while the priest and ministers pray their prayers? And why likewise does a long prayer "for the... Church militant", and the Anglican penitential prayers (with rubric to the effect that the Confession shall be said in the name of all by one of the Ministers "either here, or after the Priest's communion"), appear likewise inserted after the Secret? Is the lay worshipper to pray at least the intercessory prayer – nice, but strangely omitting explicit reference to the Pope and the Saints – while the priest performs the offertory?
Why, too, are there two versions of several Prefaces? why again does "We do not presume" (the devout Prayer of Humble Access) and the 1662 Anglican Prayers of Consecration and Oblation (definitely heretical, as regards the defective epiclesis) appear before the Canon of the Mass? but otherwise, all is as in the Roman Missal – and from Te igitur to the end of Mass, all is given in parallel Latin and English, even the rubrics (and this in a missal for the use of the laity!): strangely, while the Latin says Papa, the English coyly says "Chief Bishop". Before the prayers for when Holy Communion is distributed at Mass, and then the Leonine Prayers, there are two further Anglican inserts (for the rubrics refer back to the main text): the Prayer of Thanksgiving is provided as one of the Postcommunions, and "The peace of God, which passeth all understanding..." is given as an alternative form of blessing.
To summarize, I think I could take along The English Missal for the Laity to an Extraordinary Form Mass quite happily, and if I gave it to a friend he might not even notice that it wasn't Catholic, but Anglo-Papalist. (Another now-Catholic friend in Hobart was brought up very High Church Anglican, and went to All Saints, Wickham Terrace, in Brisbane, where the services were so High they even used Latin, and worshippers were advised, should they be unable to come to All Saints, to go to R.C. Mass rather than slum it in any other Anglican parish. As she said, when she became a Catholic "All I had to change was the name of the bishop".)
In comparison to The English Missal for the Laity, that is, the Knott Missal (and others like it), the new Ordinariate Mass, with its larger retention of Anglican forms, and use of readings and prayers from the modern Roman Rite, is actually quite moderate!