I've never heard of eunuchs in Britain in the same way there were eunuchs in China, Byzantium etc. Was it a thing? If so, when- when did it stop, when was it introduced? If not, why did some countries have eunuchs and others didn't? Thinking of the theory about the Pardoner in Chaucer being a eunuch.
It depends on how you are defining "eunuch." Did any castrated British males exist, for sure they did, castration is more common than people care to think about! But did eunuchs as a class of people with their own roles in society exist, on the level of the Byzantines and the Chinese courts, no, as you've already noticed. But they knew about castration of humans, and eunuchs in other cultures, and many visited! I have written a little about Grand Theories of Eunuchs for why there are eunuch traditions in some cultures but not others. And I repeated some of it the other day actually, come to think. Eunuchs are so hot right now.
So in the mod-tagging system (which is half good notes believe it or not!) you've actually been tagged "flair watch - England" -- are you interested in how castration was understood in early modern England and masculinities and at all? Because I can point you at some interesting books!