The main post is here at Ye Olde Resource Post.
It has multiple links on writing in general - grammar and punctuation (18 links) to style and tips (15 links, plus a link back here). There are resources on Britishisms (language and culture, 40 links, plus links to two comms on the subject). There's medieval history, with an emphasis on Arthurian legend (20 links, 6 of them Wikipedia), but also multiple non-Arthurian medieval references (15 links), and even a few links on canon for the BBC Merlin fandom (only 5 links, though, so still incredibly useful and relevant for non-Merlin writers).
There is an additional resource post, an extension of the previous one (and as such a link to it can be found at the bottom of the previous on) - Ye Olde Resource Post - Adult Content Section NSFW NC-17.
In includes a history of homosexuality (4 links, mostly about medieval Europe), British swearing (2 links), resources for kinks (4 links), and of course multiple links on sex (15 links, about 7 of them slash/gay-male concentrated).
All in all, it seemed worth reccing, at over 100+ links (133 in general, plus five more specific to the Merlin fandom). :D Hope you find it as helpful as I have!