I never go to all these "Coffee Shops" we have many around here to pamper to the poor old students, went once total waste of money.
Buy the cheepest supermarket coffee going, if I have visitors I may buy a small jar of Kenco.
Remember with supermarket own brands compare the jars, if moulding on one is the same as a named brand there is a good chance it is the named brand inside. Having worked in the food industry years ago I tell you most supermarket own brands are known brands in supermarket packaging but cheaper. Tesco, Sainsbury, Morrisons, etc don't have their own packing plants for these things they just put them out for tender. M&S and Waitrose own brands will be good quality known brands, the rest own brands will be slightly less good quality known brands. That's why many packets say made under licence for Tesco / Sainbury, etc.
Increasingly I find I don't like the taste of instant coffee whatever the brand. However I did once try buying an own brand as they only had massive jars of Nescafe in the supermarket I was in, and it was like sawdust.
Had a friend that used to work in the KJS plant at Banbury and he implied that most of the main brands were actually all KJS products anyway, just with different recipes.
As to the Coffee Houses. My favourite by a long way is AMT, though they mostly only do takeaway outlets at Train stations.
I quite like Starbucks, though they are a bit "Big brother" and the blend is rather weak, I think it tastes OK. Service is usually very good.
Cafe Nero is good tasting but a lot of their outlets seem to suffer from dire service.
Costa is rubbish. I will drink it but its pretty thin. Some of their franchised outlets in filling stations have no clue how to make the coffee!