In Scotland, they use Edinburgh Craigentinny for most services. Trains run ECS from there to Glasgow Central and Dundee. This is, I believe, NXEC's main HST depot. The depot can do quite a bit of work on both the HSTs and Voyagers if need be.
1 train every day starts and ends at Aberdeen, so one train probably uses Clayhills as well for overnight stabling. Clayhills is an NXEC HST depot, stabling two a night in addition to turning around a portion of the Sleeper. But it does little more than cleaning and only light maintenance. The XC service comes all the way up from Plymouth, and must go back to Penzance the next morning, so it is probably in need of a fuel before it goes back down, and perhaps a clean. Otherwise, they probably won't do much else at Clayhills.
They used to stable one a night in Polmadie, Glasgow. However, now they just run a train ECS from Craigentinny to Glasgow Central for the 0600 departure. XC no longer use Polmadie, but Virgin use the depot for all Scottish based Voyager operations. Four Class 221 Virgin Voyager trains stable at Polmadie each and every night.
Can't say much about South of the border, but I hope I've been able to help you with their Scottish operations.