Sometimes I don't understand people on here.
"I have an interpersonal problem that could and quite probably would be resolve in moments if I simply spoke up, but I'm British/socially awkward/too nervous/too shy to speak up. What can I do?"
Well, nothing. You either speak up, or you put up. I'm sorry if that's harsh, but you came on AIBU presumably to ask if you were BU. I think you are. There's a simple solution, it just involves taking yourself out of your comfort zone for a minute.
He doesn't know there's a problem, so he has no reason to modify his behavior. Instead of silently seething, or passive aggressively putting up ridiculous signs, or posting a very cowardy-custard note under his door, what on earth is so wrong and so hard about simply saying in a pleasant, polite fashion, "Can you please keep it down? I'm finding it really hard to concentrate." Because at the moment, you're essentially asking him to read your mind, divine that you have a problem, and modify his behavior, all without you giving any indication at all. How reasonable is that? (Hint: it isn't, at all.)
OP, I used to sit and seethe, but I'm not made for that, and so I would eventually snap and throw the most revolting, juvenile tantrums about these kinds of little problems - exactly the kind of problem that I could have solved by speaking up politely in the first place. I've had to learn (and if I'm honest, I'm still learning) that speaking up is the far better method to deal. I think you need to, as well.