With 200mm and wanting action your best bet is the chicane at the end of a lap, there is normally something that happens with the btcc here.
The spectator area here gets very busy, very early on race day........that should tell you something!
If you wander out to the complex you can try out you panning on the way, just before, and as the cars clear the rise just before they break for the complex should give you some clean backgrounds to play with.
But, before you get that far, the exit of the first corner at the end of the large catch fence gives you a good shot back to the corner, nice first lap shot with the field coming round and maybe the prospect of contact. This shot is dependant on your skills as you maybe shooting straight at the sun, hang on Easter at Thruxton scratch that bit about the sun ha ha, but it's worth a look.
With 200mm shots at the complex will be a stretch, should be ok on the entry but the rest you'll want a bit more reach I think. There is a gate that the marshalls use on the exit of the last corner that might be worth a vist, the background here is a bit cluttered if memory serves but you can get on the contact in the middle left hander from here if there is some.
They are opening up the spectator banking further round from here but as for snapping from, I've no idea, but you can bet it will be fast as from the complex exit to the braking point round the other side of the track it's flat out!
HTH
Shaun