Yup but they sell a lot of rubbish as well. Stick to known brands. Usually I get my air and oil filters from the dealers and those parts aren't too expensive either. I get many other parts at the local general car parts store or on line (like eurocarparts). Especially the stuff BMW buys and just moves on while more than doubling the price. OEM suppliers: Beru/Bremi for ignition wires, Bosch for most spark plugs (good alternative would be NGK).
Have a look at our
Preferred Suppliers List as well, many UK addresses in there. You're not the first one that wants to do maintenance on his E21!