I live in Greater London now, but on my first two trips over here with a girlfriend, we stayed at the Vicarage B&B just off Kensington High Street.
http://londonvicaragehotel.com/I just checked the reviews on Trip Advisor and it still seems to be holding its own.
Superb rates, brilliant location, and great for walking, shopping, etc., - all the things you mentioned. It's also quintessentially 'English' in ambience.
You are about equidistant from both the Notting Hill tube station and the Kensington High Street tube station. Easy walking distance to Notting Hill or Kensington High Street for shopping, cafes, pubs, etc.
You're literally around the corner from Kensington Gardens - another great place to sit and chat and people-watch.
Afternoon Tea at The Orangery at Kensington Palace is one of the best tea bargains in London.
I think this area of London is great for a girlfriend's getaway.
Hope this helps,
Denise