I go to London to visit a old friend in the spring! We have talked about it for many years, but I finally go soon! Her flat is small, so I think it would be more comfortable to stay in hotel room. She live near Soho Square, so do you have nice hotel advice that is close to Soho Square or in Soho district? I want not too cheap, but not too expensive too, something in the middle that is nice.
The Z Hotel Soho is interesting: it's the first affordable hotel with luxury finishes in Soho. It does this by compromising on space, so consider if you're happy having limited floor and storage space. Prices start around £85 a night, but this is just a starting price.
Soho is a very small (and very cool) neighbourhood. If you cross north over Oxford street into Fitzrovia (called Noho by some locals) or Bloomsbury you will be a five minute walk from your friend's apartment and will be able to get a four star hotel from around £100-£120 per night depending on the time of year and how many days you're staying.
I'd recommend considering a
Radisson Blu Edwardian Hotel (such as the Raddison Blu Edwardian in Bloomsbury) because you'll have the reliability and luxury of a respected hotel chain, but you won't be paying for luxuries you don't need.
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Actually, the hotels in London are not cheep, but you can try to stay at
Premier Inn. It's clean and comfortable, and it's location is good.