Congrats on the wedding. We had a marquee reception in a field not far from Gatwick (Oh, Ok, amongst ruins, v romantic, not just a field with grass and mud!!).
Our marquee provider also did catering and provided chairs tables etc. They were called Super Event (Google them). We highly recommend them. The food was excellent, the service was brilliant. All in all a top event.
As for somewhere to put up a marquee - have you spoken to the events organisers at the medical schools? (Guys and St Thomas's & Kings - from memory their students have annual balls but not sure if they are held on site or not).