Hi Steve,
I don't think they 'could' legitimately stop you but am pretty sure that they will try to anyway. Might be an interesting year for ship photography on the Thames, especially in the upper reaches close to the Olympics and H.M. the Queen's Diamond Jubilee celebrations.
I'm going to 'keep calm and carry on'. After all, I've been photographing ships on this river for the last thirty or so years and have been paying taxes all that time which will go towards funding these events so will take a picture or two for the album if I like ;-)
Best Regards
John J.