What are the laws on "foam blasters" in the UK?

I've been modding up a longshot recently and the laws in the UK are... fuzzy. All I can work out is it can't look like a gun, and can't seriously injure someone. But there are laws saying it has to be under 1 Joule of force (130-140 fps) but others sources say it then becomes an "air weapon", in the same category as airsoft. So what can and can't I do, as well as what should I do to make sure that I don't get into trouble in the first place (noise, etc).