I would choose the Mercer Street. The location is excellent - central without being totally swamped with tourists or on a main street. The neighbourhood has copious places to eat and drink and plenty of sights are walkable from there. Fallback options would be the Bloomsbury or Berkshire. Kenilworth is OK but have had a couple of meetings there and found it a bit drab.

Mayfair is dull and not central. Canary Wharf is a rubbish location for tourists as there is little there. Leicester Square is rather loud and full of drunk bridge and tunnel types which makes the Hampshire a bit unappealing.