Oulton is probably the best circuit in the country for spectator side shooting, although some new fencing has gone up over the winter that has taken away a couple of good spots. In terms of walking, it's more than Donington as the circuit layout isn't as compact, particularly if you want to head over to Knickerbrook and down towards Brittens and Shell. It's worth it though, it's a beautiful track, far prettier and more photogenic than Donington. I'd get most of your photography done on Saturday, and go on Friday for practice if you can as it will be absolutely rammed on Sunday for raceday.
As Simon says, Druids is a great spot in the morning, no debris fencing and you're close to the track, there are a few different shots you can get from there, such as these. All taken from basically the same place, within a minutes walk of each other
British GT Championship Oulton Park by
Richard Crawford, on Flickr
British GT Championship Oulton Park by
Richard Crawford, on Flickr
Working further around the circuit, the inside of Lodge and into Deer Leap also gives some good shots, again no debris fencing at all:
British GT Championship Oulton Park by
Richard Crawford, on Flickr
There's a bridge there, so nip over to the other side of the track and you can get some good shots along the start finish straight, again no fencing
British GT Championship Oulton Park by
Richard Crawford, on Flickr
If you can get around Old Hall into the camping area there are some good shots to be had here as well, not sure if you can get around there on a BTCC weekend though.
All along the Avenue and into Cascades is fence free and gives you some great options, as here
Head over the other bridge towards Knickerbrook and the whole of this side of the circuit is basically fence free, loads of good options here too:
British GT Championship Oulton Park by
Richard Crawford, on Flickr
British GT Championship Oulton Park by
Richard Crawford, on Flickr