Dec 7th and 21st are two dates we are planning on having a Fly-In at Old Warden. I realise 'fly-in' is incompatible with what you have in mind. . .
. . . but accommdation is often available in the house. Not sure about the room/party type thing, you would need to ask.
Centrally located, just off the A1, and the Kings Cross mainline just a taxi ride away.