I maybe wrong but I thought a friend of mine who is a BA pilot said that it is a requirement on BA for there to be pre-departure announcement from the flight-deck. IIRC, there isn't a set brief but mentioning the seat-belt policy is mandatory. I always thought that a mention of the no-smoking policy should be a requirement but evidently it isn't.
Only once on BA have I not heard an announcement from the flight-deck prior to push-back. The crew said they had decided to do it during the hour-long wait in the departure queue at JFK instead.
Personally I like the announcements from the flight-deck. It gives reassurance and also dispels some of the divide between crew and passengers.