The configurations of the aircraft all stay the same regardless of the season. The configurations are as follows:
B757-200 - 235
B767-200 (Longhaul) - 274
B767-200 (Shorthaul) - 290
B767-300 (Standard) - 328
B767-300 (Premium) - 315
I have operated P&O cruise flight with both cabins - P&O decide who sits in premium if there is an upgraded cabin.
Premium configured aircraft are normally solely longhaul aircraft. Some do operate european routes but only when another aircraft is tech (maybe wrong on last point not sure!)
Hope that helps !
BYCREWLGW