For what it’s worth my general musings are:
Ian is a relatively common name over here, but Nigel is much less so. Many of the names you mentioned would be thought of here as aristocratic, and would not be common in most communities. Nigel is definitely not a football hooligan’s name!
Julian and Jeremy I’d say are effete, middle or upper-class names, and I didn’t know any until university. I’ve never met any Ruperts, Clives, Cyrils, Crispins, Evelyns, Percivals or Sheridans and they’d have got you beaten shitless at my old school. Jemima is upper class, Emma and Charlotte used to be but are in general use now. Here’s why American places are called Charlotte.
In mentioning school, I should point out that I’m from a Liverpool Irish background, so “typically British” names would’ve been more unusual where I grew up than in some parts of the UK. Irish names like Aiden and and Keiran were fairly common, though, as were “Catholic” names like Tim, Dominic and Adrian. We also had a Venantius, who wouldn’t have survived without seeing the funny side of it himself.
The British “last name as first name” names quoted by WordMan is pretty unusual here too - in fact I’d guess most British people think of that as an American affectation. (Pride and Prejudice wasn’t written recently you know
). Naming kids after places is unusual outside of Yorkshire in my experience.
Simon is not unknown here, but it’s biblical so should be just as well-used in the US no? Seamus is surely unheard of outside of Ireland or Irish communities (means James of course).
Henry is unusual among adults here but a few kids were called after Prince Harry (real name Henry). The only English “Hank” I ever heard of was Hank Marvin, b. Brian Rankin. We don’t call Charles “Chuck” either.
Gaelic or pseudo-Gaelic names (Liam, Callum, Sean, Siobhan) have become increasingly common in England even among people with no Irish or Scots connections. I’ve got a Liam, a Siobhan and a Niamh in my family, but I’ve already told you why.
One very British name not listed so far is Graham (Scottish spelling Graeme), but if you want to hear really ethnic names head for Wales. Trevor has already been mentioned (original spelling Trefor) but there are dozens of others used there that will sound very strange to American ears.