The white clover and gorse can be easily found- I live near lots of woodland, I'll post you a load of them if you'd like.
I have seeds from some oriental leaves - they might not be able to compete against all weeds, but if you saved the seeds, you could make sure they kept growing back.
I've got Dragons Tongue
http://www.realseeds.co.uk/mustardgreens.html --right at the bottom
which is a mild Mustard green, and...
Tsoi Sim -
http://www.realseeds.co.uk/orientalgreens.html
which grows really fast - harvest in 4 weeks. it grows delicious leaves and edible buds- a little like purple sprouting.. well, kinda.
hairy bitter cress seeds are ready, but I don't think land/water cress seeds are ready yet.
Jack-by-the-hedge seeds are good to grow for salad leaves- nice taste- and you can grow them nearly all year around.
I'll swap some of all of the above, for some Nine Star Broccoli (perennial) and Organic Artichoke seeds if you're interested.