Can't help you much with gardens to look at since it seems that you are wanting a community garden rather than a garden that people can visit? (I can suggest many of the latter) You could try contacting
The Agroforestry Research Trust I think Martin Crawford has a database for Forest Gardens that may be worth a try. I found
Social Farms and Gardens as well, but many of these are not Forest Gardens, but it is easy to search for ones close to you on the maps on the website, maybe they can help if you contact them.
This
project in Oxfordshire looks very interesting :
Sustainable Blewbury but they describe it as a community orchard rather than Forest Garden. I remember there being a community Forest garden outside
Centre for Alternative Technology near Machyllneth, but It's been several years since I visited there, so don't know what it's like now. There's a demonstration garden on the way into the Eden Project in Cornwall (introduced me to Shepherdia Candensis)
but I wouldn't describe it as a community garden...
As regards practicalities, I think it probably depends on the site and the neighbours. You always get someone who'll spoil it for everyone, but if the idea is to
encourage people to take free fresh food stealing the crop isn't possible! I think I would prefer keeping dogs out (mine are sometimes quite desctructive) - could it be combined with a kiddy area if all the plants are carefully checked (you'd want to do this anyway I guess)? Then the children are fenced in and the dogs are fenced out. Forest gardens can make great places to play.