butty is a controversial one which means different things in different parts of the country (like for me a bacon butty would mean bacon in a burger bun, not a sandwich) - you can definitely use sandwich and I would stick to it for safety
you can still say cupcake, hungry and dessert and be perfectly British
cozzy is short for swimming costume, which is what I’d actually say
I’ve never heard a scarf being called a muffler
You can use all of the “American” insults here and still remain British-sounding apart from jerk because no one says jerk. Some of those “British” insults sound a bit silly to me (please don’t call someone a plonker dear lord).
Don’t ever say cheerio unless you want to sound like a twat
Eyewitnesses claim that the Grinning Man is a looming figure who stands well over 6-feet tall. His head is said to be devoid of any hair and his dark, beady eyes are nestled unnaturally far apart in his domed skull. Some observers are so perturbed that they are later unable to recall whether he even had a nose or ears, but — as disturbing as that may be — far and away his most disconcerting characteristic is the wide, hideous, shark-like grin that is perpetually plastered across his face.