There's something which could be done which would do both of these things but actually reduce costs.
It's also something which
@Irish Rail would probably like.
I've talked about it before on other threads.
Basically you lengthen about 1/2 the 5 coach sets at GWR and LNER to be 9 coach sets, most of the other half of the 5 coach sets are then sent to XC.
However, you can keep the number of coaches on lease at GWR and LNER the same by retaining +1 × 5 coach set for every 5 sets which are lengthened.
For example, the 36 × 5 coach class 800's at GWR require 180 coaches and create 18 full length equivalent trains (I.e. either a 9 coach set or a 5+5 train is full length).
If that were to be 15 × 9 coach sets and 6 × 5 coach sets (165 coaches) GWR could also have a further 3 × 5 coach sets and the number of coaches they lease would still be 180 coaches.
Only the number of full length equivalent trains is 19.5 rather than 18. That doesn't sound much until you remember that's basically extra capacity for almost free (subject to lease costs being the same when adjusted for inflation).
Then the extra sets from GWR and LNER can go to XC and even in a 1:1 coach swap XC gets more capacity with lower running cost (and just the reduction in track access charges could allow the same number of coaches as getting the stored 221's to be in service with XC but needing to attract far fewer passengers to make the business case work).
Whist it could be complex to buy the extra coaches which are added to a leased set you could probably do a coach swap, so some of the new coaches become leased but some of the old sets become owned.
The lease company would be happy (those new coaches will likely cost them less in maintenence over the term of existing contract than the original coaches) and GBR own some sets.