HolidayCheck is a really good site where you can read hotel reviews! Then you can look at the cost vs how decent it is!
Check out Spa and Pampering, for the list of London's top spa hotels. You can also see what's the best London can offer on terms of spa services and hotel accommodations.
Yeah I'd recommend the Crestfield, which is a hotel near King's Cross. Cheap but pretty central and very friendly.
Yes, In my opinion you can go with http://www.montcalm.co.uk/ for Boutique hotels London
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