Go early, especially if good weather is forecast. It's a park wherever you like sort of thing. You can hunt out Clubs/any group that puts up a banner, but if you time it wrong you can spend too much time trying to drive round and find them, or end up pissing everyone off by trying to drive against the flow. Best just to get in and park up.
Anything Austin, Morris or Rover goes, regardless of where it was built (I bet most attendees will think it all came from the hallowed ground across Lowhill Lane, anyway!) Oddly sniffy about people trying to get Land Rover, Range Rover and early MINI products in, though.
As already mentioned, being early April, the whole show is subject to short notice cancellation, which is fair enough as the organisers can't risk ruining the park or the Council would never agree to host it again I should imagine. People seem pretty clued-up for organising short notice pop-up-POLs in the local area. Check social media on the day. Popular alternatives are the British Motor Museum, Gaydon and The Transport Museum, Wythall.
I'm planning on going (come rain or shine - I have a Premier Inn booked!) in either or both of these, depending on a second driver;