Try Peak Imaging (http://www.peak-imaging.co.uk/
). Base price for E-6 processing, mounted, is currently £5.90 per roll. They offer pretty good discounts for multiple rolls, too. I usually wait until I have a cache of several rolls built up, in order to earn the discount (and save on postage).
The quality of service is excellent, and you'll normally have your film back within a week of sending it off to them. I'm well pleased with their work, and use them regularly (since the utterly lamentable demise of dlab7.com's brilliant E-6 service).