Really interested in the new Junction studio too!
I've done regular yoga for many years and hot classes at Yogahaven in Clapham Common
http://yogahaven.co.uk/clapham/ which is a really lovely place. They're not a Bikram studio, they have their own more varied styles, including a hot flow class, which I really enjoy - I gather that Bikram is very static and is exactly the same script for every class.
Your first class will probably feel very intense, I felt nauseous in mine, the important thing is to stick with it and stay in the room, even if you just lie on your mat for the whole class the first time you go. In time, you just forget the heat, if you are focusing properly on the poses and your breath. You don't even notice the sweat rolling off your skin when you really get into it. It's lovely in the winter to spend 90 minutes at 40C!