every word containing, "th"had an "f" in place.
This is called Estuary English and is very widespread. You wouldn't expect all crew to be schooled in RP, and I find the mixture of accents quite charming, though occasionally when there is a delivery in Glaswegian at the speed of a machine gun I do wonder how easy it is for a non-native speaker to follow. But last night wasn't about the accent, more about the incoherent delivery.
Incidentally what happened to the taped announcements in the language of the country you are flying to/from? I'm sure this has been discussed here before but it only recently occurred to me that they used to be heard on all BA flights and are not anymore. I'm sure they were highly appreciated by passengers back in the day.