What a beautiful project and a beautiful setting. Congratulations on your efforts.
From my own
experience, I have used
WWOOF, Workaway and HelpX as a volunteer. With two exceptions, out of more than 30 projects, I have felt very welcome and left wiser and with very positive memories. I only ever stayed for 10-30 days but I know some people that use those services stay with hosts for 6 months or more - it is, of course, up to the host and volunteer to decide.
You could consider listing your project on those services if that appeals to you. The mindset varies for different volunteers but it seems that most fall into one of three categories:
1. those looking for a way to travel cheaply and have an "authentic" experience of a place
2. those wishing to learn about
sustainable agriculture/forestry/etc, often with a mind to setting up or joining a similar project themselves
3. those who want a break from city life
I'm sure there are many more reasons too, but that is a broad (and opinionated!) categorisation from the people I have met.
You could also consider reaching out to projects in nearby cities and offer them use of your space. They could then pass this information on to the people involved with those projects and they might help with the admin and even the insurance.
In London, I know there is a
Permaculture Network, a 'Permablitz' and countless smaller projects such as community gardens, foraging groups, herbalism, natural dyers, etc. I'm sure that many of the members of those projects would appreciate the opportunity to visit a project such as yours and feel welcomed. Whether there are equivalent projects closer to you is another question.
Concerning insurance, we have public liability insurance for our land for claims up to £2,000,000. It costs us ~£250 each year. We don't have a large number of visitors but it does mean that we are protected in the event that someone does hurt themselves and decides to sue - or if a tree falls onto someone's car, for example!
If you are hoping to have members of the public on your land regularly it is something to consider and weigh up the cost/benefits of. As I mentioned earlier, you might be able to receive a contribution to cover insurance costs from those benefiting.
I hope there is something of use there.