I am 10uk in Carmina Oscar jumper boots, and they are roomy. Vamp is not closed when laced tight with thin socks. I am also 10 in C&J 337, 348, A&E 99, 104. I keep it pretty simple, I am even 10UK in Simpson.
Interested. I have to figure out Rain. In my size in Simpson, I had a nice snug elongated fit. The vamp was almost fully closed (my low instep). Comments on the instep on Rain? I have Oscar boots and they fit well. Maybe rain is oversold as 'wide' but more like normal as opposed to Simpson.
I am one size smaller in AS oxfords than the AE 5 last. If your AE5 is loose, and you prefer a snug fit (as a loafer), go 1.5 sizes. IMHO only, I have AE5 in 11E (too wide) and 11.5D (too long), and I am wearing 10UK in AS 104 and 99 lasts.