AutoEurope.co.uk
Have always used them when in the UK, I don't even bother with the industry rates now as 90% of times it's cheaper. They compare all the main rental firms plus some local too. Make sure it's the UK website though as this includes insurance.....the US one doesn't!
Get yourself an international yearly insurance policy that covers car rental excess for around £50 per year and you can avoid paying overpriced excess amounts too.
Harry