Visit London have suggested this, like it very much. It will be fun, arty and noisy. Best idea so far.
“The atmospheric Café in the Crypt under St Martin-in-the-Fields church in Trafalgar Square is serving traditional Christmas lunch and dinner – turkey and all the trimmings – from 13 to 24 December. The famous Trafalgar Square Christmas tree is just outside, and all profits go to support the charitable work of St Martin’s.”
I think it is ‘self-service’ though which will be a little weird!