Depending on where you are going, Priority club may be the best option for getting points. (IKEX3 is also another rate that is often useful, particularly at weekends). 25,000 PC points will get a decent hotel in London; 15,000 will get you a HIX somewhere near (e.g. Stratford, Croydon).

You can probably redeem whatever points you have in London without any problem, but outside London the options are more limited, and often not good value compared to cash. 40,000 points might get you the Doubletree Regent St, but it will take 35,000 for the Birmingham Metropole (£60ish). Here, PC will probably give you the most options, but city-centre hotels are often 25,000 points, even for a HIX.

There's not many Hamptons in the UK, but a few of them are available for 12,500 points (Liverpool, Derby, Shrewsbury, I think).

Lastminute.com have 'secret hotes', it seems to be where some of the London Hilton and Radisson Hotels (amongst others) sell off their surplus rooms - you can usually google the description to find out what hotel it is. (e.g. I have stayed at the Hilton Paddington for around £70).

For smaller hotels, Laterooms.com is probably the best bet online.

Travelodge hotels often have sales on, particularly when it is quiet (like December!). They are very much a no-service chain, but are usually reasonably well-located. (e.g. no included parking, early check-in, late check-out, shampoo and they don't have luggage storage or telephones). When they have a sale you can get rooms for as low as £10, even some in central London. They are often reasonably priced if non-refundable rooms are booked well in advance.