I've just come across your post and, like you, have found tennis incredibly beneficial. Unfortunately I live in Leicestershire otherwise I would love to help you set up a group.
I started playing tennis a few years before I was diagnosed in 2011 at the age of 52, and it would have been so easy to give up at that point, but I carried on going to our local club three times a week. For a long time my condition deteriorated but I battled on with the tennis. Our club is quite healthy and there's always someone there to play doubles with.
Then, just over a year ago, a few things changed for me. I stopped working, I made an effort to put on weight (I'd lost a lot of weight over the years since being diagnosed without really realising it) and I started taking Madopar, as Mirapexin seemed to be making my condition worse.
I now play tennis nearly every day and my game has slowly got better and better. I even played a few matches for our club in the county league this winter.
People are commenting on how my game has improved. You don't always have to hit the ball hard, placement and strategy can win you the point.
Like you say it ticks all the boxes and it's moving without thinking and such a relief from the difficulties I have the rest of the day. I honestly don't know what I would do without tennis, it keeps me going.
Good luck with setting up your group. I hope others will give it a go.