Big Walkies. Most of their events seem to be concentrated around St. Ives, where the main organisers live, but they do list some walks in Milton Country Park, and I would imagine they may schedule more this side of the county if they get enough interest.
Membership is free (they request a donation of £1 per dog per walk to cover admin costs), so this may be an option for people who can't afford formal dog training classes but want to encourage their dog to develop a positive attitude to other dogs.
They have some sensible Terms and Conditions which walkers are expected to comply with:
(Disclaimer: as always, the RSPCA can't take responsibility for events that are organised by someone else).