Can't remember doing this is London (though i'm sure it woud be very easy) but snuck into Act 2 of Beauty and the Beast in Melbourne a number of times while I was backpacking there after seeing the full show. Can say that I've seen Hugh Jackman's Gaston quite a few times because of it (though not the performance where he wet himself). And worked out the Beast's transformation.
We DID used to interval upgrade at Les Mis back when it was at The Palace though as the £5.50/8.50 Upper Circle seats gave you a great view of any empty places in 2/3 of the stalls. Good luck trying that in Harry Potter though..