I find Toby carveries quite handy - roast ham/beef/turkey then you serve yourself with low carb veggies and can easily avoid the potatoes. I then ask for butter in my local one and have that instead of gravy.

Chinese buffet places are manageable, but only if you have iron willpower as the vast majority of meat dishes will contain buckets of sugar and cornflower. Usually there will be some plain fried chicken somewhere though.

Any indepenant cafe or greasy spoon that does fry ups. Usually it's quite easy to just get bacon and eggs without the carby beans and sausages etc. Supermarket cafes and chains are often nowhere near that flexible though.

Kebab shops - all our local ones do kebab meat with salad in a tray - it's even up on the board.

Nandos is very handy as the menu is pick and mix. A chicken salad is easy to get, and the coleslaw doesn't contain dairy (at least the last time I asked it didn't - the staff had a folder with the ingredients for everything on behind the counter)

Greek restaurants are very good for grilled chicken/lamb and salad. I've also done tapas a few times.

I find in most independant or posher english style food/pub chains that you can ask for no potatoes, or for salad instead of veg, and so on. Not so easy in the cheaper places though.