Blue-sky thinking here: Could the surplus MK2/3 coaches be converted to push/pull rakes with 1/3-2/3 door arrangements like what New Zealand did with some of their imported MK2s?
Could it be done, yes.
Is it likely to be done, no.
It is unlikely to happen due to the cost of such works compared to useful life span when compared with a new build, especially given the need for DVT's or DBSO's and the cost of running a loco.
To put the cost into perspective the track access charges of a 180 (the highest of the 1xx classes stock) is 14.03ppm (pence per mile) per coach, compare this with one class 43 (HST loco, which is one of the cheapest) which is 37.35ppm then each coach (mark 2 as the are a bit cheaper than the mark 3's) at 4.75ppm.
This means that a 5 coach 185 is marginally more expensive than a loco and 5 coaches (with 4 coaches being the other way around) - although that is assuming that DBSO were the same cost as their coach counterparts.
Change it to 8 mark 3 coaches (6.46ppm each) and two class 43's (basically a HST) and the cost per mile would be £1.2638 compared with a 9 coach 185 (I know they don't exist) which would be basically the same cost at £1.2627. Likewise a class 222 would be able to be 11 coaches long and still be a comparable cost at £1.2111.
I know track access charges are only one element, however given some sets can run for a thousand miles a day, even small savings can make a difference to the running costs, sometimes enough to offset higher leasing costs.