Hello Barnsley;
Welcome to GMIC! I'm not entirely qualified to answer your questions (to begin with, I don't live in the UK), but I can give you some basic advice. While much of it will not be that remarkable, it may help reinforce your suspicions on collecting.
There are many reputable sellers, but with any item you consider you should have 100% money-back guarantee from the dealer. When you are starting out, you want to buy only from reputable dealers. Just like a buying a car, look through all the inventory available in the marketplace. Compare and contrast what you see. Seek out as many examples as possible and look at them closely. Take notice of their construction and handle them where you can. Until you build up a confidence from seeing and handling many "good" pieces and known "bad" pieces you can be susceptible to the Fakers' latest and best output.
Go to the military shows and visit all of the dealer tables. Don't monopolize their time, but ask them questions about the pieces you are interested in; condition, materials used if paint or enameling, the base metal, what is rare, what is common.
Knowledge about the piece and how, when, where it was used will not only help you identify a good piece and a good deal, it will further your enjoyment in collecting.
Good luck and good hunting...