I work in a Post Office and I can tell you that you can use them. They are worth the value printed on them, and if you add two half pences together they make one pence, so to use your example a 12 1/2p and a 20 1/2 p would make postage of 33p.
It does not matter that we no longer have 1/2ps, we had a missive from the Post Office last year confirming that two 1/2 p stamps make one 1p stamp!