British Men and Jewellery
First off, let’s notice the spelling. This is the British English spelling, not the American version ( which is jewelry).
Secondly, it isn’t a thing over here to the same extent it is in America. In England in particular, jewellery is a signpost to class (isn’t everything?)
Rings
Aristocratic males seldom wear wedding rings, for example. Prince Philip didn’t, Prince William doesn’t. Charles does (Camilla probably insisted, LOL). BUT, take a look at where Prince Charles wears his wedding ring- stacked (almost hidden?) by his signet ring which he has worn since the 1970s.
Signet rings are symbols of aristocratic origins, so think of engraved family crests. Just initials? That’s a clue to a poser, someone not quite of the right lineage. No jewels involved; no diamonds, no cameo or onyx.
Often worn on the little finger (which is rarely called pinky/pinkie except in Scotland and America)- and on the non-dominant hand. My version of Watson wore one for about twenty years before a swollen knuckle put an end to it. John Watson, being left-handed, would wear one on his right hand. But middle-class/working class men wouldn’t wear one (because they don’t tend to have family crests). Also, as a former surgeon, he would not have been allowed to wear one at work. Let’s add in there the fact that the British army isn’t keen on jewellery being worn while in uniform or in training, with the exception of a wedding ring or a signet ring.
So, what else would a British character in your fan fic wear?
Cufflinks are also a class and profession signifier.
Brash bling is something you’d see on a City dealer or investment banker, or someone in the music industry. For someone like Mycroft, the cufflinks will be antique, gold and have some sort of engraved design on them. No jewels, no gimmicky images. Always doubled, no stick and swivel back(which is modern).
These are made for tailor-made dress shirts with French double cuffs. They can be plain gold ( as these above seem to be) or engraved with a design as these Victorian set are.
Then there is the watch.
For Mycroft there can only be a pocket watch. A half hunter or full hunter. A Half Hunter enables you to read the time with the cover closed and usually has a window in which you can view the time through whereas a full hunter has to have the cover open to read the time and usually has a sealed closed cover to help protect the watch face. My guess is that Mycroft would have a full Hunter, that is, a closed one. It would be part of his signalling his aristocratic origins to complete the ritual of taking it out of his waistcoat pocket and opening it.
Sherlock wears an Rotary watch, the Canterbury model. It’s Swiss and under a £100. Mind you, give how often he dashes about, gets thrown into the river and other assorted rough treatment, an expensive watch would be pointless.
Then there is John’s Watch. Now THIS is a surprise! The Tag Heuer Monaco watch costs over £1,000. So John may like to dress down, but his taste in watches is rather high end!
So what else?
Not much, to be honest. It is not usual in anyone over thirty to wear studs, earrings, bracelets or other jewellery. Oddly enough, thirty year olds and younger might now do so, but it would be likely that they’d take it off anytime they went for a job interview, because it is still seen by society in general to be not normal.
So, fic writers who have British characters in your stories. Be aware that in this culture, men’s jewellery is rather understated.