That reminded me. If you'd prefer to depart from LHR they should also arrange that. They should let you change the flight to depart from any of the LON airports they depart from.

It's also worth asking BA if they'd contribute towards any necessary hotel costs, although from LGW you can get there pretty much 24 hours from Blackfriars, I checked a random weekend and weekday date in July and that's still the case, unless there happens to be engineering works, but we'd need the exact date to check that.

Back many years ago when KLM cancelled my LCY flight and moved me to LHR as the subsequent LCY flights were full I finally convinced them to pay for the hotel as I said the reason I chose LCY was so I didn't need to get up ridiculously early to get to LHR, but then they changed my flight only a couple of weeks before.