Hi Sam,
You remind me of a past version of me, who got into permaculture, and especially forest gardening, not long before our first child came along (who is now 13). That said, I live in Canada, which is a pretty different place, though we did live in Wakefield, UK for 8 months (in 2002-03).
One of the most difficult, but also most transformative things I learned in the early years was to accept, and eventually embrace, the principle outlined by David Holmgren of "Seek Small Slow Solutions". It was actually something from Martin Crawford that really got me. When trying to establish my first forest garden, I had 'weeds' come in. A major one was creeping charlie/gil over ground (which is a ground cover here in Canada, but is originally European, I think). In one of Martin's books, he actually had it profiled as something that was used in the past to flavour beer, as an alternative to hops. It's not so much that I planned to make beer, but something about even this plant that my brain had strongly categorized at the time as a weed could have a usage really started me on the path to where I am now, which is to accept the help of whatever weed-friends that show up as much as I can, and only try to diminish them so they can collaborate with whatever I want to nudge the place towards growing. (Ie. Keeping grass short near young trees.)
In terms of actually speaking to your question about securing land to work with, here are a bunch of ideas which may or may not be of use to you. Many of them may have a flavour of seeking small slow solutions, which is one of my favourite flavours.
1. See how you can get involved right now, in terms of your capacity to commit. Whatever you are doing, that has something to do with that path towards a forest garden, can, and likely will, be helpful. Be prepared to be patient. Then when you get there, it will actually probably be somewhat surprisingly casual. "Oh, right, this used to be a dream...now it's reality." At least, that's the way it's sometimes been for me - from an apartment dweller in Toronto, to now living on a 50 acre permaculture farm. Something I did early on was get involved with a local Transition Town. I worked on my skills at facilitating meetings, and working on a committee - which does not sound like forest gardening, but was part of the mix. This can also be a way to meet some folks. The one we were in had a few seniors/pensioners in it - see below.
2. Seek out people who are willing to share their land for your forest gardening practice/projects. Though I have ended up planting trees and spending large amounts of labour on land I no longer live near, or even have access to, as long as you are fairly sure they won't get cut down immediately after you are involved, it can still be a good thing. Leaving the fruit trees at my previous home 7 years ago was very hard, even though we sold to a herbalist who would enjoy them, let alone the dozens of trees at a farm that's far away from where I am, and the trees are probably not even being noticed by anyone. They are still there (the ones that made it). Ways to find people who might have land to share (or eventually gift, or sell to you) include getting involved with clubs of like-minded people, and driving around and looking for 'neglected' areas, such as disused lots, or people with larger estates that look sort of 'run down'. There's an idea from Joel Salatin that has stayed with me, which is to look for neglected pastures (ie. filling slowly with trees), and knock on the door of who owns it. He said, in his experience, it is often a little old lady, who would love to see the pastures used for cattle again, but there's no one in the family to do it anymore. Obviously in the UK this is going to take a different form, since there are not just acres of pasture lying around, but look for signs of land that someone is not 'doing' something with, but is not really a functioning natural area either. You may want to look at Paul Wheaton's forum pages/book about SKIP, which is about gaining the skills to care for land that someone may want to see if they are going to let you work on their land.
3. Get used to making lots of mistakes, if this is new to you. This is not a bad thing, but it's okay to feel bad about it, especially when you are getting started. I've lost more trees in a year than most people have planted in their lifetime. It's just part of it. Observe all the things that you think may have gone wrong, and look for solutions for the future so more of the trees you plant make it. For me, this includes planting the trees where you can casually observe them often when they are getting established, and most importantly, putting on tree guards the same day as planting (every time!), and having really great communication with tractor operators working near your trees.
That's it from me for now. Best of luck on your journey.