MML have on order 33x5 bi mode trains (class 810). Why not ask Grant Shapps (DfT) to add on to that order so that XC current rolling stock can be replaced.
There's two problems, firstly any XC units would be delivered after those for the MML.
Secondly, this would likely lead to a slower rate of improvement in that the 222's can be delivered to XC during delivery of the MML batch if trains. As well as requiring less staff training time to bring into service.
The 222's would bring significant improvements to XC, as with just the 17 x 5 coach units that more than enough to run all 4 coach services as at least 5 coach services and all the 5 coach services to be run with pairs of units.
I'd be inclined to use the 222's for the Manchester services. By maths I think that it would work.
Bournemouth to Manchester takes 5 hours and Exeter to Manchester takes 4 hours, as these services are inter-worked a unit is on an 11 hour cycle.
This means that to run all services in pairs (or equivalent) you'd need 22 units with 5 coaches. You'd then need 3 to act as spares/for maintenance. That would mean you'd need 25/27 units of they were all 5 coach units.
However the 222's aren't all 5 coach units. Yet we could get close to them mostly being so.
If we shortened 2 of the 7 coach units to 5 coaches that would give us 23.
If we then shortened 2 of the remaining 4 7 coach units to 5 coaches and lengthened those last 2 units to 9 coaches we'd get our 25 x 5 coach units which we need to run the Manchester services as a self contained fleet.
Now that doesn't show for the services to Paignton, but does extend every service to Exeter so there's scope to stop some services short at Bristol to allow those few services to Paignton. However this would still be a better service provision to Exeter than the was. Although if the services were to split there could be 5 coach services South of the split, which could allow a full hourly service to Paignton without needing any extra units. (By my maths I think the spilt could happen at Taunton, which would still be a better service provision and much higher capacity for the whole route).
That would still leave 2 x 9 coach trains to strengthen other services, in addition to the circa 18 trains released by the introduction of the 222's.