If you want something that is a bit different, how about Skinners Hall in the City?
It's old and pretty, you go through a gate and little tunnel and find yourself in a lovely courtyard, a real little oasis and not what you are expecting. It's one of the Guild Halls - home of the worshipful company of skinners. Also has a fountain roof garden and beautiful indoor rooms.
And it has a fab address - 8 1/2 Dowgate Hill
It won't have the spa ambience of a spa/ hotel but you definitely have a feeling of stepping off the street and into another world and time... Definitely serene, with that sense of calm and away from everyday life.
They are used to doing weddings and corporate events. I had my wedding there - that's how I know about it! But everybody who went to the wedding was really impressed and surprised (nicely!) by the venue. And as it is in the City it is very convenient - very close to Cannon St station etc. And as venues went, I seem to remember that they were very reasonable price-wise and the food was exceptionally yummy. ah, happy days .