For the right price, sure. But it'd have to be a meaty enough operation to warrant dipping your toes into the market north of the border, some way from the existing reaches of the empire.
If you assume that First/ Stagecoach/ National Express are keeping their current networks, and that Lothian Buses aren't being privatised any time soon, then that really only leaves McGills - West Coast look a bit too "niche" IMHO.
The problems with McGills are many - they seemed to spread themselves too thinly with the Lanarkshire/ Dumbartonshire expansion - the owners appear "characters" - the Inverclyde and Monklands area still seem "bandit" territory... you might be able to reach some kind of "peace" with First (I don't know what relations are like between the two around Hampshire, which is about the only place that the two have overlapped in the medium term?) but it might be slim pickings.
You might be brave to enter Scotland whilst politicians are making noises about regulation, though, that said, they did buy First out in the city of Manchester.