Are you looking for green beans? or already roasted espresso?
I don't know any place in the UK, but if you don't mind shipping overseas some good places to check out would be Stumptown, Intelligentsia, 49th Parallel, Alterra, Anodyne, or (my personal favorite) Metropolis. I can't guarantee that they'd ship overseas (freshness issues...) but you could always try!
Not sure where you are, but I HIGHLY recommend Larry's Beans. (www.larrysbeans.com) based out of Raleigh, NC. All FTO, all amazing. We used his stuff exclusively at a shop I worked in there, and I still purchase it even though I live in Texas now.
Larry runs a great place on the last unpaved road in Raleigh. I recommend Secret Espresso #17. Also, side note - the Malabar XXX is a dark roast coffee that is amazing.
He sells both roasted and green beans. Larry has also visited his farms, which speaks volumes.
Are you enjoying Barista Exchange? Is it helping you promote your business and helping you network in this great industry? Donate today to keep it free to all members. Supporters can join the "Supporters Group" with a donation. Thanks!