My fiancee and I are spending part of our honeymoon in London and we'd like a really great, romantic hotel to stay in. Price isn't as much an issue as location--we'd like something pretty centrally located if possible.
I really love St Ermin's Hotel . It's a gorgeous boutique hotel, beautifully restored and easily accessible to all of the London (a 1 minute walk to St.James Park). And they make their own honey from bees on the roof!
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The Met by hyde park is cool. And staying there gets you into the Met Bar, which can be fun at the weekend.
The other thing would be to enjoy some amazing english countryside outside of london, somewhere like Clivedon House. Not far from london. Amazing place.