can I stick 2 first class stamps on an A4 sized envelope?

[toomuchtodo]

or do I need a special stamp on it now??

Was told by a colleague whose husband works for the Post Office/Royal Mail/whatever-they're-called that it has to have a large letter stamp, else the postal police come and take you away...

if it covers the cost of postage then yes.

ye swiil bed fine

You can but it is a waste of money.