Disclaimer: I’m not from the UK!
“Grim” - I’ve heard of the word obviously but I’ve never ever heard anyone actually say it to describe anything. It’s a favourite here!
“Naice” - I’m still scratching my head over this one. What does it mean? 
“Pants” - I’m going to assume these are undies/knickers? Otherwise some conversations I’ve read really don’t make sense 
Also, what is half term please? I’m gathering it’s some sort of holiday half way through the school term, how long do your terms go for? Ours are only 8-10 weeks (4 terms per year) so obviously don’t need holidays in the middle..
Teach me all the UK things!! 
I had a whole list, I’ll be back when I remember the rest.. 