My piggies love nothing more than a romp about on the grass, when it's warm enough, and mine has plenty moss! The pigsters don't bother with the moss (loads of yummy grass to munch!) They also enjoy clover - though have only fed in small quantities, as don't have too much to give! Have always given dandelions in moderation - as believed it to be a diuretic, overworking the kidneys, but see it raved about here. Let me know...
Never tried ''sticky willy'' (how unrefined!) /cleavers - though have plenty of it. Will give it a go.
Would be wary of offering oxalis as there are loads of cultivated varieties and not sure all are safe. Infact 'wood sorrel' in the UK has white flowers not yellow as in the picture. Have tried it myself, and it is pleasantly sharp. Would still steer clear for my piggies.
Avoid anything that grows from a bulb. IF IN DOUBT, LEAVE IT OUT!!!
Would also welcome a list of safe UK wild plants.