Keep in mind that United allows a free stopover on an award ROUNDTRIP--So, no free London stopover on way to Spain if you book two one ways.
IMHO, BA avios are good for "free" flights from UK to Continental Europe. In fact, one of the few decent uses for avios. Might be time to get a couple of BA cards from Chase.
IMHO, waiting until a trip is being planned to start looking at miles cards is not so good an idea. This is long term thing to build up miles balances to can fly in Biz or F.
And, as mentioned above UK departure tax is a killer. Remeber, you don't pay the tax on flights to UK--only on flights out.