I just tip to the nearest pound for taxi rides. The only time I use taxis is to get from the city center (ok, fine centre) to the university area which should be about £4, so I always give a fiver. If it comes out to exactly £5, no tip. If it comes out as more than £5, I’m a bit upset and again no tip because it shouldn’t cost that much. So, basically I just tip to the nearest pound mostly because I don’t want a bunch of shrapnel (I’ve never written that word before - looks quite strange written).
For take-away food - I’ve never tipped. It would seem very awkward and they certainly don’t seem to be expecting it. I hope I’m not upsetting them - but they seem to be friendly enough to me. I have no friends who tip for take-away food either. But, we are all students, so perhaps our views on money should be taken with a grain of salt. (That, and I’m not actually from the UK, so maybe I am constantly making social faux pas without knowing it)
There are one pound notes in Scotland as well. Yes, they are terrible - makes it look like you actually have some money in your wallet only to realize that, in fact, no there is not enough there for another beer.