If it's with a different service provider they won't let you do this (maybe you could beg but I don't see how any company would allow this as far as they are concerned they have your £100 now). You could keep the sim for that and use the credit with that sim, but if you have a locked phone it'll not work with a differen't network.
We wouldn't be able to take your credit from the other network I'm afraid! If you contact them though, they might be able to have this refunded to you, given the high amount.