Totem Hobbies.Local shop to me in Plymouth.Kevin (shop manager) has built up a really good line of quad related items.http://www.totemhobbies.co.uk/First Person ViewGood shop and fast delivery. As it's name suggests, more for FPV related items.http://www.firstpersonview.co.uk/QuadcoptersNot bought from these guys yet, they have a good reputation on the net.http://www.quadcopters.co.uk/HobbykingEveryone knows these guys http://www.hobbyking.com/hobbyking/store/index.aspBangoodFrom China, so slowish delivery times. Good for spare props, batteries for the small quads like Hubsan etc.http://www.banggood.com/Wholesale-RC-Quadcopters-c-1848.htmlEbayBought from these guys in Ebay. Mainly props and cables etc. They send out pretty quick, UK based.RC Spareshttp://stores.ebay.co.uk/sparesrc?_rdc=1Pyramid modelshttp://stores.ebay.co.uk/PYRAMID-MODELS?_rdc=1Orange RXhttp://stores.ebay.co.uk/RC-at-Factory-Prices?_rdc=1I used these got my naza m lite from them stevehttp://www.fast-lad.co.uk/store/ these are okay but limited stock at timeshttp://www.unmannedtechshop.co.uk/
just contribute another website: https://www.allbuy.com/list?cid=RC-Quadcopters-288another china online retailer, some quadcopters there are really cheap.
I have always bought my quadcopters from RED5 - http://www.red5.co.uk/remote-control-toys/quadcopters.aspx I have never had a problem and their selection is decent enough as well.
There is also Heliguy for some of the more expensive stuff - http://www.heliguy.com/