I don't think there are any existing Royal Navy surface mods for SH3. But there is a good Royal Navy submarine mod called "We Dive at Dawn". It' s for Silent Hunter 4 only. It has all new British submarines for the player, plus British uniforms, sounds, and campaign sequences:
https://www.subsim.com/radioroom/sho...d.php?t=170639
Here is a video of a youtuber playing the mod:
I hope this helps!