I don't live in the UK, but I did a lot of messing about with RTV silicon moulds and resin casting last year, and I can confirm that they're pretty easy and a lot of fun to work on. Great for making your own bases, terrain elements, or anything else that you need repeated in quantity.
I've only used the thick-liquid-with-hardener type. Not too difficult to use at all. Give it a go, you'll be happy you did so. The folk on the forum over at
http://www.resinaddict.com/ are super friendly, and they can answer most moulding/casting questions.