We could consider Peterborough. The Town Hall has meeting rooms of
various sizes available, up to a couple of hundred.
For a further short hop by train, or shared taxi (with a 7 seater it is
reasonably cheap) Stamford has a lot to see and many eateries, including
the famous George coaching house, the grave of Daniel Lambert rooms for
hire in the Arts Centre, the town hall (possibly free
http://www.stamfordtowncouncil.co.uk/index.php?id=20) .
Both towns are on the A1 and accessible by various transport, although
the Harriers only stop a few miles away at Wittering and a short walk
would be needed, the hot air balloons depart Stamford but do not arrive.