LHR is expensive, but a good two day trip. There is a hyatt which is fine:comfortable but nothing like the rest of Europe. Don't know SPG situation.
I think MAD or BCN has food (check the master threads, there was a lot of recent discussion about hashgacha)
AMS also sounds cool, never been
If you've never been to LHR I would probably go there. A ton of food. Gotta see the crown jewels, etc. Dan would tell you to go elsewhere