For English girls via agencies, there are three main places who have good girls with decent reputations - MaxesAngels, Hamiltons Escorts, and Bijoux Escorts. Maxes are better value than the others - Blair, Abigail, Belle, Jessie, Abbie are the English girls I would recommend from there (two are reviewed by me so far)
Hamiltons are more expensive - I have had some great punts with Rachel - but at £250 an hour it isn't the best value.
As others have said, you will get a better deal going independent - recent reviews on the forum will give you an indication of who's high on our wish lists.