another that seems to be very scarce these days is Greengage; ten years ago I had "picking rights" in three or four different places and now they are all gone. Greengage is a delicious and very underrated fruit and very versatile.
There are loads of Greengage in the west midlands/worcestershire and yes it's nice and often overlooked, certain parts of the staffs/worcs canal has a large bounty of them along it's towpath. Another nice one is the Mirabelle Plum tree, i know of a few growing wild in hedgerows and the plum is really nice to eat fresh, it also makes fine jam and its used for making plum brandy or wine.. I also like the John Downie crab apple tree, despite being a crab the little apples are fairly sweet and nice to eat raw, they also make an excellent jelly.