I agree about the awful Burger Kings in old Little Chef outlets. There's one at Skipton which is absolutely awful. Disinterested staff, disgusting toilets, and despite it being very quiet when we last went in, it took over 30 minutes to serve our simple burger and fries meals. There was just a couple of staff shuffling around behind the counter and in the kitchen area seemingly not remotely interested, just chatting away, disappearing and then returning a few minutes later (presumably fag break). And then, after all that, the fries were stone cold!
I've found motorway service station outlets of BK to be pretty good. Usually pretty busy but they seem to actually get on with it, usually pretty quick service and hot (ish) food. We live not too far from Burton services (near Carnforth), and rather go to a "proper" BK in Lancaster or Morecambe, we go the back way (via A6) to Burton services instead. There's also another good one on the M62 near Leeds (I think it's called Hartshead Moor) who we've also found to be good. However, having said that, the Scotch Corner one is absolutely appalling.
But we definitely try to use BK rather than McDonalds these days. Used to like MaccyD's but they've really gone downhill, with slow service, cold food, etc - they've sold their soul to Just Eat and Deliveroo, whose drivers take precedence leaving "normal" customers waiting whilst parts of their order goes cold.