Hi,
I travel to UK every year from Zim, and I've had good deals from the following (typically about 2/3rd the price of dealing direct with the car hire companies).
Having said this, I've just asked for a quote on these two web sites and the cheapest rate (Ford Ka) is 765 quid!!! Probably cheaper to buy a car while you are in the UK and then sell it at the end! You can get secondhand cars quite cheaply.
Good Luck Graham Carter Harare Zimbabwe