Is there a NERS scheme in you area? National Exercise Referral Scheme.
We have also found physiotheray a once off and then it's up to you, but, with the apathy that comes with Parkinson's it has to be regular,every week at least. NERS and a referral to a physio got OH on a NERS scheme locally as their poor balance was in danger of them suffering falls.
Also explore, LIFT. Limited Impact Fitness Training,. Sounds harsh but it is mainly conducted from a chair.I have organised this for our local branch, through Age UK.