I'm 55 and I've never had a jab in the butt in the UK, even during my childhood vaccinations. I think they've found the arm is good enough for most shots and it's less embarrassing for people. You don't want people not getting jabbed because they're embarrassed, we have enough trouble getting people to be vaccinated already.
Yes, most people won’t encounter vaccination in an area other than the upper arm. It is almost always used for small volume injections (i.e. a mL or so). A few vaccinations require a larger volume dose and used to be given in the bum because the larger muscle mass can absorb more liquid. But I think the hip is now actually preferred because use of the bum suffers from the risks of both damaging the sciatic nerve and a too rapid take up of the drug. My only experience of the more undignified approach was when I used to have to have Gamma Gobulin jabs from the company nurse for certain overseas trips.