We have hired this company in London for"private car" transfers
before and would certainly use again. Their website shows only stretch-limo but they have small sedans or minivans too..so whatever type of car you want them to use is available. ( depends on how many people )They do tours etc. You just need to email them with what you are interested in doing.
http://www.alphalimos.com./

Also, there is a gal who posts alot on CC ( Cheryl) whose husband owns a limo/transfer company who I had been in touch with last year ( although we did not use them) you might also see what his rates are.( to compare)
http://www.woodfordchauffeurcars.co.uk/