Silly little Brit question...

Dear friends of mine from across the pond,

We finally got to see the first episode of Steven Moffat's Sherlock Holmes on PBS last night, hosted by Alan Cummings who pronounces it "Sherlick" in all his adorable Scottishness. (Love that guy.) Anyway, at the beginning, John Watson is sitting down to a cuppa and the camera focuses on a crest on his mug. I figure it's to establish either a branch of service or his educational background, but which is it? This is the sort of thing that would be entirely lost on an American audience, and I'm sure it doesn't have any huge bearing on the proceedings, but I want to know! Does it show that he's really elite at something, or from quite a middling background, or what? The only thing I'm sure of is that it's not from Hogwarts...

Thank you in advance for feeding my obsession with trivial detail.