St Pancras has 3 - the 2 "domestic" ones both seem happy to sell anything; a bit silly to have separate ones side by side really... good old "competition" (="fragmentation").
When I last bought a ticket at Gatwick station, some years ago now, there were 3 separate ticket queues, to 3 sets of windows, and people were being told they had to be in the right queue according to whether they wanted to travel on GE, Southern, or T/L trains. So does that imply that whichever one you went to would try to refuse to sell the full range of tickets (and I guess arrivals from abroad wouldn't know any better)? But what an absurd introduction to British public transport for foreign visitors...
I didn't know Euston had 2 ticket places. When I tried to get a ticket to Milton Keynes there one afternoon a few years back, I went to the only obvious place, got to the head of the queue, and was asked - in effect - which colour train I wanted to go on. Same price, much the same journey time! I said whichever colour went next, and he faffed around and said, well, you might just make train X, if not you can't use the next one, train Y, and will have to wait an hour (or whatever it was) for the next X. So no problem with the ticket office selling everything, but the more major problem of the absurdity of not being able to just get the next train and not worry about the colour... I think that was the point when I really started to hate privatised/competitive transport systems... I ended up late for my meeting in MK.