Second Limewood, really lovely albeit quite a busy place. The Newt in Somerset is incredible value for money if you manage to get a room. Beautiful hotel with a spa that is not massive but is gorgeous and the ladies who give treatments are fabulous.
I have reviewed it on my blog and I also listed other hotels with spa not far from London (Chewton Glen, Coworth Park, Soho Farmhouse etc) on my blog, if that's useful:
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