Sorry to have gotten into this thread so late. We have stayed at County Hall 3 times, twice were on rewards (which is the best way since it costs more than $300 US per night). I don't see how you could have a better location! You are right between the Westminster and Waterloo Stations. It is a very old formerly gov't building that has been restored exquisitely.

The rooms are cheerful and bright. They are spacious (allowing a family of 4 to be comfortable which is hard to find in Europe).We didn't care for the expensive, non noteworthy restaurant. There is a McDonald's downstairs which is great for breakfast especially if you have kids.

The indoor pool and exercise facilities are terrific. The bell/concierge staff were inefficient and gave us alot of wrong info on our first stay. We just didn't use them on subsequent stays.

I don't see how you could go wrong staying at County Hall. I did notice other guests twice who had moved from Marble Arch because they thought County Hall was so much more convenient.