Is there anywhere where cheaper theatre tickets can be obtained for, say, a last minute trip to London? Have never done this before and the prices are pretty high in advance.
there are quite a few places like this, but they're really only for same day bookings. there are loads of places like it in theatre land. are you trying to get tickets for a particular show?
as mentioned, half-price tickets are available on the day at the two sites, one in Leicester Square (it's the one in the actual square, not the dozens of ticket shops in nearby streets) and one at Brent Cross shopping centre in north London. They aren't available for every show in town, of course, just ones with unsold seats, so if there's a particular popular show you want to see you might have to bite the bullet and book in advance at full price.