Well, it seems that Apple don't do a 1:1 conversion like most American manufacturers seem to do (especially software), but of course it's not 1:2 like the £/$ relationship was not so long ago. Actually, currently the pound is quite weak - it's 1:1.7 at the moment.
The model they've got right now - 1099/699 = 1.57. I think that was released at around the time we were at 1:2.
Based on that, if they keep it proportional with the weakening of the pound, I predict somewhere around 1:1.3 this time around, which would make the $799 model £615.