I actually emailed the brewery about availability of Okell's and was told that Sainsburys and a handful of pubs are the only places to get it outside of "the island", ie IoM. Tho with Sainsburys in NI it's readily available here.
I also forgot to mention Meantime London Pale Ale, which is probably a bit closer to GH than Okell's. Green label, it's 4.3%, not to be confused with the very much stronger stuff they also do (I think).
Tho I will say that I have a hard time telling Deuchars and GH apart.