I've been doing some research to hire a car over Christmas and seem to have come across some good rates from Thrifty.
I live in Reading and just booked nine days rental of a Ford Focus sized car, from 23 - 31 December for £126.52. I was originally looking for a week, and could have had an economy car (Renault Clio or similar) for £84.06 all in (24 - 31 December).
This is significantly cheaper than all the other car rental firms I checked, and just over £10 a day for car hire seems like a bargain to me. I tried a couple of other locations and while anything London-based seems notably more expensive, some other towns are showing similar rates.
Please note: the deal doesn't include any additional insurance over the basic (I never take that anyway), but does include unlimited mileage so there shouldn't be any extra costs.
You have to pay up front to get the best rates, but you can cancel up to a week before without any penalty.