I used LHR as an alternate for LTN this past December, I had filed LTN as our destination and Bristol as our alternate (Departed
St. Johns, NFL) the weather at Luton and Stanstead had gone below minimums so we requested a landing at LHR in order to try and save our pax a drive from Bristol. ATC said the diversion was OK as long as we had ground handling aranged like Mike Jenvey stated. It took a bit of a scramble with the Acukwik and the sat phone, but ATC was very accomodating (It was 11pm) and as soon as we acknowleged we had handling we were cleared for the approach.