Great White Shark in UK waters?

Marine biologists believe that the waters around the UK are at a temperature that Great Whites could easily live in. Add to this the fact that there are many seals around the UK that are a staple part of the Great White's diet then this makes there even more of a chance that Great Whites visit UK waters. So why has there been no concrete evidence produced such as photographs that prove this theory ???! Please feel free to comment on this topic.