I agree with exmodlad Serg, save some money and plan your trip from here, look for cities and locations in order to see where are you going, what's close to it, how far from the airport will you be travelling.Plan hotel expenses, taxi cab expenses, etc.It sounds like a lot of things, but I'm almost shure it will be cheaper than having someone pre- select a vehicle for you and charge you for that.A good thing would be searchiong through auto trader-UK by putting certain cities on the search engine, for that you will need to know city post codes so it's better to sit with a map in front of you opened in another window of your computer, then look for codes through the UK postal service website, then you can play around with some cities so you can have an idea what's on the market there, the approximate prices , etc..It may take you a whole afternoon, but it can certainly save you some money.Just my opinion.
Thanks,
Carlos