If you’d rather poke your eye out with a drywipe marker than spend the day sitting in an airless room with no daylight, you might find this list of London venues useful.
Big, open, well lit spaces help create a good atmosphere for great meetings.
Creative spaces (such as rehearsal rooms) are less expensive than London hotels or ‘destination venues’, there are many good options for that kind of space here: rehearsalstudioslondon.net