http://www.pricepoint.com/
http://www.nashbar.com/bikes/TopCategories_10053_10052_-1
http://www.cambriabike.com/
http://www.performancebike.com/bikes/TopCategories_10052_10551_-1
also a national chain ala bikehut, they bought out Supergo who used to be the US equivalent of CRC when I lived there years ago.
http://www.speedgoat.com not the cheapest, but have a great rep for knowledge and service (think good LBS with net presence) and occasionally good deals.
If youre mail ordering into Uk then plan for +20% VAT, +import tax (varies on whether component or bike IIRC is 4% for parts), handling charge by shipping company (typically £8), and possible lack of warranty. If youre bringing it in personally I think customs have bigger concerns than a couple of bike bits (may have to pay local sales taxes if, like Performance, they have a store in your state). Ive brought stuff back on trips and have yet to be stopped, theyre more interested in people from certain destinations with hand luggage only (drugs, terrorism).