Gilbert Syndrome
Philosopher
I don't believe any of those Great White sightings have been confirmed, however, have they? Until one can be confirmed, I'm going to lean more towards mistaken identity involving mako, porbeagle or basking sharks. Porbeagle and mako are in the same family as white sharks, and fleeting glances could easily lead to false identification.
No, they've certainly not been confirmed, although there's been a few which have been thought to be genuine, but there's not a whole lot that can be done without solid proof.
There's certainly room for mistaken identity, but I don't doubt that White's could very likely be visiting these shores during the Summer, as it's not too far away from the Mediterranean, and there's plenty of seals and such for them to feed on.
Here's a good article concerning the whole thing: https://www.google.co.uk/?gfe_rd=cr&ei=QquiWPHPHcf38AeupKeIBA#safe=off&q=great+whites+in+uk