To most users the railway is the railway and they have no reason to know one company from another. It's something railway staff deal with on a daily basis.
Indeed, there's a lot of poor understanding about the railways, I overheard someone saying that nationalism wouldn't work as it was only the running of the trains that had been privatised and not the track or the trains!
The planners are constrained by the funding available. The government subsidy for XC is 2.6p per passenger-km, compared to say EMR at 3.6p or TPE at 12.9p/P-km.
On those numbers I'd argue that you could cut the XC subsidy further by running longer trains as your staff costs (roughly 1/3 of a TOC's costs, but it's likely to be higher at XC) would remain the same if it was a 4 coach 220 or a 9 coach 80x. Yet, whilst you'd have to cover the other 2/3rd for the extra coaches, you could put in some small reductions to make it more attractive to go by train.
Let's say a 4 coach 220 costs 300, to run an 8 coach 220 unit (yes I know they don't exist) would cost you 500. However with double the number of seats, you could aim to increase the number of passengers by 80% (from 200 to 360), that would mean rather than charging an average of £50 (and £5 of subsidy) you could charge an average of £40 and pay a small premium to the government (about £2).