They were never going to get 31/33 into traffic every day, that was pure fantasy, presumably designed to meet some or other target.Reported in Modern Railways a while back that the original plan was for 31 out of 33 unites needed, hence the (now discarded) plan to keep the 4 x 180s.
Realistically, you'd plan for around 28/29 out of 33 units. The base timetable requires 19 units, so that will leave you with 9/10 for strengthening, so around 50% of services could be 10 car, assuming that the sets are used all day and not just at peak times.
There will still be an increase in capacity, the 27 x 222s and 4 x 180s being replaced by 33 x 810s.
Assuming Wikipedia is correct...
The 27 x 222s have a total of 1618 1st and 5206 std seats .
The 180s have around 160 1st and 900 std seats.
The 33 x 810 have a total of 1551 1st and 8382 std seats.
And south of Kettering may/will get 12 car 360s at some point.
They may be longer vehicles, but the Class 810 at 254 standard seats is barely better than a Class 180. Nowhere near enough for a major route out of London. An 8 car EMU is be well over 400.
They never learn - TfW and Northern bought brand new fleets of 110 seat 2 car units.