I use Londontown.com I always get the Millenium Gloucester or Bao;eys, for under 100 GBP . Excellent location with good transportation options. The Breakfast is more than plentiful . I usually purchase Club Privileges for an additional 10 GBp , it includes breakfast, Internet access, afternoon tea , cocktails and plentiful canapes in the evening. I skip the room upgrade as the rooms there are large!

Last year, in early Feb, Hotels dot com had the superbly located 5 star Royal Horseguards on a 48 hour sale, with substantial discounts pushing room rates down to around the top of your range. Can't remember whether it extended to Sep/Oct and no idea whether it will be repeated, but worth keeping a lookout.

I LOVED the Royal Horseguards. I was only able to stay there once for 3 nights--got greatly reduced rates through a friend who was a travel agent. Never stayed there again because regular rates are way out of my price range.

www.booking.com/hotel/gb/dolphin-house.en.html
May fav in your price range for no bid other good ones too.
Betterbidding.com for great deals bidding priceline.com
GREAT 4 star values there from $80/nt for me if not too picky.

Good luck!

This was a timely post for me as I also have a trip planned for Sept. and have been a bit shocked by prices of London hotels - I don't like to pay more than $200 per night but think I am going to have to up my pricepoint a bit for they type of hotel I like. The Sanctuary Hotel looks good but I am wondering about the neighborhood - near Westminster - will it be a bit quiet in the evening - more like mid-town Manhattan than one of the neighborhoods like the Upper West Side? I want to be close to good, inexpensive eats (ethnic restuarants) or wine bars. Thanks for the input.

Suec1,
The Sanctuary looks appealing. I used Google Street View to look at the neighborhood and it might be quiet on weekends and evenings. It's a block from Westminster Abbey and around the corner is a tea shop and a restaurant. It's close to the Underground. Photos on Trip Advisor show some king sized beds so my DH would be happy.

Someone from Fodor's referred us to: [email protected]
We gave some guidelines...price range, logistics to train etc.
We found a lovely flat near Victoria Station for @150 USD. This was several years ago. But the bottom line is that the website offered several viable options. We were thrilled with our accommodations and could walk to any number of famous London sites.