I know this is not geographically accessible to a lot of people on this site but it really deserves a shout.
There is a dedicated Parkinson's spin class on Sunday lunchtimes in Clapham Junction. I've been twice and there are 6 classes left of this batch. A group of us are trying to get it out there in the hope that more classes can happen at different times in the week. It's possible to start at any point during the 8 class block...
The instructor is specifically trained in PD cycling and there is plenty of scope for differing abilities and help with anything. I have a spin bike at home but this is my first experience of a live class.
I will say that it's great fun. So much more than exercising alone. I work harder than when I'm alone and I look forward to it (!).
Cycling is supposed to be particularly good for us...
PM me if you'd like details...