Not in the UK, but I still shopped at a UK store to get my Avon tires faster than I could get them in Finland. Good customer service and the prices were also, so that might be one way to go.
Since the UK is still in the EU, you also have literally all of Europe at your disposal with minimal hassles.
One large player there would be Louis Moto | Clothing, Helmets and Motorcycle Accessories and I've shopped there also, reasonable prices and shipping and decent assortment; they have some "house brands" of clothing that's very reasonably priced. I picked up some "Proof" rain pants there for next to nothing and they worked just fine, kept my lower body dry during 8 hours of Norwegian pouring icy rain.
Edit: oh great, revisiting M&P caused me to blow 200 on new boots and some cleaning stuff. Oh well... needed the boots.
Edit 2: Forgot to add, too obvious I guess - for Triumph specific stuff, World of Triumph has everything you need.