IME of at least monthly on the route and almost always on the A321 highly unlikely that you will not get proper short haul C seats. I've only seen converter seats once in the last twelve months - they are not what you hope for but better than most other similar seats by a country mile.

Sharklets do not indicate whether it's proper C or converter, the TC-JM*s have proper C as do the TC-JS* and TC-JT*, it's the TC-JR*s you have to watch out for but they rarely appear on London or even UK services.