In Sydney there is a HUGE coffee culture - and 99% of the coffee shops are independents.
Starbucks does exist (there is maybe 5 of them in the whole of Sydney), but it's only frequented by brit/american tourists. There is also somewhere called "Gloria Jean's" which seems only to attract tramps.
The reason that Starbucks etc has failed to take off here, is because the coffee from the independent shops is significantly better. Seriously, it's amazing.
There is also an intangible piece about "experience"..... why the hell would I want to sit in a supermarket and drink a coffee? Not exactly a deluxe experience is it? Would I sit in a supermarket and drink a coffee, even if it was free? no, probably not.
As far as I can see - if the independents in the UK are a) offering better coffee, and b) offering a nicer experience/environment, then they have got nothing to worry about. If they can't offer better than a supermarket (not a very high bar), then they are probably not a very good coffee shop.
Don't mean to sound harsh - but there it is.