I've been told (and contrary to my previous belief!) that Crosscountry separately diagram 220s and 221s, rather than just chucking out one or the other indiscriminately; seeing as you work for the company I'm sure that you know more about it than me: In that case, if all Crosscountry's 221s were to be lengthened to seven carriages by the addition of two pantograph cars, or one panto car and a trailer, then all of these sets could be fitted with a more substantial buffet counter, and be formed:
DMF - MRFB - PTS - MS -(P)TS -MS - DMS
For the record I don't and haven't ever worked for CrossCountry!
I don't doubt that they diagram them separately, you see many more 220s than 221s at Doncaster as they work the Newcastle-Reading/Southampton services for instance. However my understanding is that there are a number of occasions where a unit terminates on one service group, then forms the next service on another. I think the difference is that with a combined fleet there will be off peak times where its convenient for a 220 to form a SW-Scotland service, and vice versa, which becomes much more difficult if you're operating 2 separate fleets.