LHR seems very hit and miss when it comes to strollers at the gate, the first time i flew with my daughter to the US on the return trip VS took my stroller on board because they said they couldn't guarantee i'd get it back at the gate. The last time i flew BA though they returned strollers at the gate :/
I've experienced the same at LGW, one of the many reasons i invested in a Trunki
Had the same at JFK, TXL, DOH to name a few with strollers coming out with main luggage/over sized bags not just in the UK