I've seen them often sold at the weekend farmers' markets. Last I saw them was the Sunday Marylebone market (which is a small one so I assume they're more readily available at larger ones)
You can sometimes buy it here dried but it’s normally sold out.
I'm in Newcastle / Northumberland but I dare say as you are in the capital you'd be able to find a suitable independent retailer somewhere. Newcastle has a place on Chilli Road and given the size of Newcastle compared to London, you'll be able to find somewhere.
I'd probably start with businesses that sell organic food. But you already probably knew that and so these words are futile. I offer them in good faith.