Dealers
Rental Cars are also available from many dealer locations in the UK, Often dealers will allow short term renal of cars and vans from there depots thought the UK for the lowest rental costs I would personally recommend a specialised car rental outlet over a car dealer anytime. However this is not always the case so you should check out your local dealers to get the latest prices. I would also recommend checking out contract hire cars for longer leasing periods as some dealers have some fantastic offers on. Especially Ford branches as ford seem to heavily back there franchised UK network with incentives for units out on lease. This means many sites will undercut one another in order to shift the most units. But then dont even get me started on brokers as they have even more competitive targets than the car dealers so in order to shift more units than car dealers the brokers will sell you there granny as part of the contract hire rental package. She even comes with free two year servicing parts and labour!
Anyway, don't ignore car dealers when looking for rental offers, check out some local dealer groups first then some national ones online and im sure you will find a bargain.
For Contract hire and leasing vehicles check out brokers and online leasing companys, they seem to have far better deals than dealers as they buy cars direct from the manufacturer using fleet buying power (that dealers don't have!) then sell to customers at heavily discounted rates.