Of course you can still use Christmas stamps that say 1st or 2nd on them. In fact you can use ANY stamps that say 1st, 2nd, Large 1st or Large 2nd on them, no matter when you bought them, they are still valid for the service it says on it. And you can use any stamps since decimalisation, so if you've still got a couple of 12 1/2p stamps lying around you can stick them on an envelope, though you'd have to make the postage up to the correct amount with other stamps.