It's truely awful for photos, and I'm going from experience on a day when there were hardly any people there, when it's busy it must be even worse. BTCC runs on the National Circuit, so you have two options really...
Luffield - A few things you can try on the terrace, stand back and you can just about get some panning shots in or them just coming around the corner, or you can stand down on the right and get them going away from you.
Copse - Head to the infield and go up in the stand on the inside of Copse, get quite a nice view of them coming off the corner.