Chris,
Sunningdale Old is a great old course, but honestly, as you regularly play the Sandbelt, I can't see it being worth it - there's not really any holes better than what you will play most days in Melbourne.
You wouldn't want to miss West Sussex, which is absolutely first class, reasonably priced by Heathland standards, and not that difficult to get to.
If you don't want to go that little distance from London, stick with Woking and The Addington - you'll get a hell of a lot more out of those two than you will any of the others.
Unless you can wangle your way onto Swinley Forest, of course.