Not sure, there is something about burger joint where they cook the burger fresh in front of you, hard to compare a burger place where they just do burger and a pub which chef will do a lot of different dishes. They could just serve you Iceland frozen burger and chips on a plate and it taste good after a few pints. I have been to some pubs which serve really nice food from scratch and good value, not many in London.
I usually ask if they use a catering burger, made inhouse or supplied by local butcher and will select something else from the menu if a catering burger.
Not sure about London as our eating is mostly from Kent to Dorset with visits to Canary Wharf and places we usually eat at are already known for their food and the like and would loose business fast if found to be using Iceland burgers..