Thanks OFSO,
I knew that the site itself was breaking the law in the US, but it's actually the legality of viewing (and not downloading and storing) in the UK that I was interested in.
Filmon is still going at the moment and I do use it when working abroad, but one of my workmates has queried whether or not actually viewing the programs is illegal. Personally, I don't think it is, but if I am wrong, I was just wondering what the specific offence would be as I don't store or copy the copyrighted material, which seems to be all that legislation such as the copyright act covers.