Is it possible to go anywhere interesting abroad, buy the maximum duty-free amount of stuff allowed and then sell it in Britain to offset the cost of the flights, and what would I have to buy from where to do so?
Well the max duty free is the old 200cigs + litre of spirits etc etc, and applies to non EU countries, can't see you getting very far with that. I think you really mean "can I bring back enough bootleg stuff to pay for my holiday if I sell it?" Yes you can bring back as much as you like for personal use, but legally you cannot sell it on. (from EU of course).