I don't drink coffee so I base this on how every chain's version of a "hot chocolate" is:
Costa: very dependable, very nice, solid 8/10
Starbucks: a bit too mild and not very chocolatey. 5/10. Very good frappucinos though.
Cafe Nero: the best of the big brands. hits the spot
and I don't think the others have a "stamp" free drink scheme? 10/10
Greggs: cheaper than the rest and...it tastes like it. too sweet. 5/10.
Pret: seemed fine the one time I tried it, nothing special. 7/10.
I do not know of any independent stores nearby but I think "Boston Tea Party" is a Devon/Cornwall chain and theirs is excellent too. 10/10.