Forget the UK, try Amsterdam or any of the major German airports. I would recommend Dusseldorf, as being easy to reach, Frankfurt of course - again not hard to get to, but also try some of the smaller airports such as Hannover or Nürnberg.
The UK is one of the most spotter unfriendly countries, which is somewaht ironic since the hobby is very much a UK one - but then again, as the UK's motto says "security is paramount" - can't imagine it's not in other countries, but they do manage to balance security with public access rather better than we do in UK.
Where there's a will, there's undoubtedly a way - the UK just doesn't have the will.