We've taken an UppaBaby Cruz to the gate at LHR T5 (traveling CW) with no problems and then gate checked it in its large travel bag. Hell of a time folding the thing but it did get in the bag eventually!

Folded with seat (as we used it) and even without seat, it is larger than BA's official dimensions but they didn't mind:
https://uppababy.com/cruz/overview/