It is safe to assume the majority of the attendees from outside Europe will arrive in London and most of those from Europe will arrive in London or the southeast. As a result logistics mean that Scotland and northern england would be problematical. In terms of facilities people are generally looking at universities so in practice we are probably looking for universities within striking distance of London.
Well, Edinburgh can be a possibility if you go north.
The accommodation is a plenty whether one looking at university related or otherwise. Flights and trains available from London. Tourist demonstrably have no problem traveling there as seen from the numbers during the Edinburgh Festival. However, as because of the festival, August will be out, so one will be looking at mid-to-end July.
There's 3 local universities that might be interested in supporting.
Cost is only marginally cheaper than London (and Oxford?).
Not that I'm not biased at all being at university at Edinburgh....
KTC