If you give dates and times I can check weather at your arrival airport. However, there was a lot of fog a few weeks back across the UK which did affect BA and other airlines and cause a lot of disruption.
captain specifically told the passengers "no matter what you have been told at the airport, we are NOT delayed due to fog. It is due to "mixing up and changing" aircrafts"
I doubt the captain said those exact words, it is so out of kilter with anything I have ever heard from a BA pilot over the PA. There well may have been an aircraft change caused by the fog as the original aircraft could have been delayed on some other journey and a different aircraft was substituted for it.