Heathrow is the standard alternate for Gatwick on all fleets at Thomson.
Might want to check the AIP on that one. You can't just flight plan any alternate and just expect to be accepted there by ATC. Unless of course you are declaring an emergency (in this case it was not an emergency, just a diversion).
See here for more info, we discussed it recently in ATC forum.
http://www.pprune.org/atc-issues/581...alternate.html 