Yeah it's all just considered a first although (not 100% on this because my work wasn't of that standard lol) I think work that's graded 80+ (???) is considered publishable? So like if you got an 80-90 mark your lecturer considers it to be of a standard that it could actually be published whereas a 70 is just a really good piece of work
I'm not sure but I imagine if someone got a high first they'd probably just say they got a high first but either way a first is really good regardless of the actual number, but people who are reallyyyy clever and consistently score high firsts are probably more likely to win awards for their work or scholarships if they wanted to do a masters, I think most departments in most unis will give out an award for the best dissertation which scored the highest mark and stuff like that