There are loads of web sites. Most suppliers have gone online as cook shops don't find it that profitable to stock the huge selection of tools, pastes, etc the really good decorators want. There are a few shops in the UK that are dedicated to cake decorating but I think most of them have online presence now, too. Your local cook shop should have some basics though. I'm pretty sure there's also at least one magazine available at WHSmiths.
Many local colleges have classes in the adult and continuing ed programs. The library is a good place to try some books without buying.
One thing you will discover, which you may already know, is that many decorated cakes in the UK, in particular the wedding cake, will be done with fruitcake. If fruitcake isn't your thing, you might want to look at the US sites for cake recipes because you do need a certain degree of firmness for these cakes.
FWIW, as I know you are heading back to the US, I'm pretty sure it's easier to do this as a home business in the UK than in the US. This may vary from state to state. I have a friend in Maryland who takes classes but can't do this as a bonafide home business because MD food/hygeine laws are too strict.