I also suspect they will have the same problems as GWR, where they curse a fleet of 5car units, and wish they had lengthened some to 7car. Yes another type, but easier to operate than a double unit (guard in wrong unit, and catering whole train difficult). 5car is too short for some workings, but 10car excessive, and mid journey coupling takes few minutes
The current order for 33 units allows for all peak services to be 10 car, plus around 50% of off peak services.
For example, from a fleet of 33 units you would realistically diagram 28 or 29 sets.
The half hourly Sheffield service requires 10 sets, and the half hourly Nottingham service requires 9 sets. So you will have around 8 or 9 sets available for 10 car running.
The 180s being retained also allows for a couple more services to be 10 cars.
The 810's will be a increase on capacity compared to a 5 car 222's so almost all services will have a increase in capacity even if they stay as a 5 car service. At the moment only 2 services join on route these being the two Lincoln starters on weekday mornings. Other than that it is at Etches Park, London and possibly Nottingham and Sheffield during turnarounds.
The morning up and evening down Melton Mowbray services also split en route at Kettering.
IIRC a five car 810 will better a seven car 222 for capacity.
5 car 222 = 50 1st and 190 std = 240 seats*.
7 car 222 = 106 1st and 232 std = 338 seats.
5 car 810 = 47 1st and 254 std = 301 seats**.
* some sets are different due to reforming the 4 x 4 car and 2 x 7 car sets into 6 x 5 car sets.
** assuming Wikipedia is correct...