It's the fact that it's considered cheap food in the USA plus in the UK it's not even table service. You're talking a few rungs up from BK at 2.5 times the price.
Honestly mate, I don't understand your fixation on table service. Most people I know are indifferent as to whether they get table service or not unless they're going for a full blown restaurant meal. There's a well established precedent for self or semi-self service for casual dining in the UK. A lot of people (myself included) actually prefer it, because we get the payment out of the way early, and we don't periodically get our conversation disturbed by a stranger expressing insincere concern for whether we're enjoying our food or not.
Beyond that, it's not 2.5 times the price of BK. If we compare burger prices alone:
Lowest:
BK - Whopper (Single) - £3.79
5G - 'Little' hamburger - £4.75 <- The 'little' denotes that it is a single rather than a double, all else is the same
Highest:
BK - Angus Smoked Bacon & Cheddar (Double) - £5.99
5G - Bacon cheeseburger (Double) - £8.75
Consider that the burgers at 5G are much higher quality and use fresh ingredients rather than frozen/microwaved at Bk. It's not a bad deal at all. I love me a BK, but they're not comparable.
As I say, when me and the missus eat at 5G we pay the same as when we eat at somewhere like Nandos. I don't see what the fuss is about.