I flew cityflyer express last week on a short hop down to the channel islands. It does depend on the route, as to which aircraft type is used. Mine was an ATR whatever with tiny seats and noisy propeller engines (think AA Eagle and you get the picture). It did have a business class, but i couldn't see much discernable difference in seat pitch - maybe a better meal - but was only a 50 min flight. The lounge access was not that great a benefit either.

Best step would be to check aircraft type on the BA on-line timetable before committing to an upgrade.