Rules may have changed since I last flew as crew but when I was at Cityflyer Express (a BA Franchise) we had to carry out the full demo on any flights over water. On the LGW-NCL route on the ATR we did not have to include lifejackets in the safety demo but on the RJ100 we had to include the lifejacket on all flights.
My memory is not 100% brilliant, perhaps someone who is ex Cityflyer could confirm this for me!
Rick