The answer to your question is "everywhere" just like the answer to the question of "I'm looking at flights from the UK, which other origin is a good one?" Things are never simple, often, ba.com will give you the best deal and booking direct has advantages (not least in case of IRROPS) but equally, many OTAs can occasionally or sometimes more frequently have significantly better fares than BA itself (other times much worse). It's not always the case and not always the same one. I've often had better deals with Omega, Opodo, sometimes Ebookers, various more obscure ones, and rarely Expedia.