what is pretentious about wanting to drink something palatable?
Nothing. However you come over as someone who wants to pretend they are better than the plebs who drink at SB. If you'd said 'personally I can't stand the stuff' there'd have been no issue.
Would it be snobbery if people said they preferred, say, Stella to a can of Tesco own brand lager?
No, it's not snobbery to prefer one thing over the other. It's snobbery to look down on people with more popular tastes though.
If neither of these things were genuinely not meant then say so, and be more tactful next time or you'll sound like nobs again and start an argument
Re SB, I'm not under any illusions, it's nothing like the best I've had. Whittards single origin coffees brewed at home are much nicer and the best I've ever had is Jacqmotte in Brussels, but Starbucks is quite drinkable. More so than Nero, Costa and Cardiff's independents. However what Starbucks do really well is sandwiches, milky sweet concoctions (caramel macchiatos, eggnog lattes etc - they seem to be the only people who can manage not to put too much cloying sweet vanilla in their vanilla lattes) and the blended ice drinks which are lovely, hard to replicate and just not as good anywhere else.
I won't defend their prices tho!