Well I'd be a bit concerned over treading on the toes of Black Park Model boat Club.
I believe they organise regattas throughout the year, although I'm not sure if they're open to everyone (anyone else like to chime in), and they are a mix of targets and subs.
I'm making some progress with enquiries over Trent Park, when (if) I get it in the bag, I'll let you all know.
How do you guys feel about a small fee for admittence, say about £5?
Here is a couple of shots of the pool at Trent Park from Google Earth, far away and close-up. The pool isn't massive 90x 30ft, so would suit small to medium size craft.