Once the doors are closed, the captain can authorise any changes he wishes so if you have any mates who're in BA it's always worth letting them know you're travelling so "you can perhaps meet for a coffee at T5." Dressing smart casual helps since some CSDs get sniffy about upgrading anyone in jeans/tshirts.
More often than not, my colleagues have only been too happy to help. Just don't drink all the champers or try to seduce the crew!