I think ts a nice touch that BA flight crew take the time to say goodbye to those of us who ride in back. This was standard practice for many years at Delta in the US in the days when they were a better airline than today and traded rather heavily on the 'friendly Southern folks' approach.
I also think its nice to say thanks for a comfortable flight or a nice landing or pleasant commentary from the flight deck and good for passengers to see they person behind the @captain Speaking ' voice from the flight deck.
A good few years ago I was on Delta trip from Atlanta to Bermuda- a sector I flew many times and on this especially nice day they descended the lovely TriStar to I suppose 1500 feet and flew downwind along the spectacular Bermuda south shore before flying a gentle curved approach all the way down almost to touchdown- a fabulous view and really nice flying experience for the passengers.
As I got off I thanked the rather veteran looking FO and asked if he had flown the sector. He was pleased someone had noticed and confirmed he had. I responded was he ex US Navy ? ( many DL crews were) which brought a bigger smile and another Yes- why had I asked? I said it felt like a carrier style approach and he grinned and just said it was great to fly to a quiet place like Bermuda on a lovely day so they could genuinely fly the aeroplane-it was a while back and I suspect he has retired now along with the TriStars -always my favourite
PB