I've competed in the WSCC Speed Series for six seasons now -- even though they changed the rules in 2007 to prohibit non-Westies competing in Westy classes (something about unfair advantage

). This year they have introduced a system of target times to allow for the fact that they run 6 classes and the average entry at each of the 30 or so events is about ten cars. Obviously, it goes out of the window in the rain (there's a special rule for that circumstance), but otherwise, it works.
Their take on classes (for Westies) is:
A/C - CECs with screens and 1A tyres up to 1800cc: ditto, B/D - over 1800cc,
E - Road legal (Tax, Insurance, MOT) CECs and BECs (1B tyres, aeroscreens) up to 1800cc and 1150cc respectively: F - ditto, over 1800/1150,
G - Non-road-legal CECs and BECs (slick tyres, no screens, wings all allowed) up to 1800cc and 1150cc respectively: H - ditto, over 1800/1150.
Those classes work too.
For simplicity, I'd suggest only classes E & F in road-going for us, and G & H. Why, because, in the Westies, fewer people each year are going in the "screen+1A" classes.
HTH