Looks like they were trying to protect their trademark and logo (the rest of section 5a) and they went a little over the top. Probably somebody with an impoverished imagination. ("If anyone creates an image link to our site, the image will probably be our logo. We must stop that! What? Exceptions? But why would anyone create an image link to london2012.com without putting our logo in the image?")
The real question is: Does a ToS like this actually have any legal force, either in the UK or anywhere else in the world?