There's a London Midland ticket booth next to platform 11 at Euston, that's the only one I know of.
I didn't notice it saying anything about only for LM tickets? We bought some tickets there about a week ago, as there were no queues and I thought I'd give them a try. Managed to get AP tickets from different origins but ending up on the same train, and sat at the same table, and with bike reservations

Very good service. Telesales at FGW could not do it, although they did try.
I have noticed the Glasgow Central ones for Scottish destinations only.. does anyone know if they have an exemption for that or do Virgin always have more impartial windows open than Scotrail have of their own? I never thought to check when I was there! Not asked for anything too complex from Glasgow apart from one problem, and they were very helpful too (that was Virgin).