According to CAA regulation 61.01.16
"A commercial pilot, when acting as co-pilot in aircraft NOT normally required to be operated with a co-pilot, shall be entitled to be credited with not more than 25 per cent of the total flight time during which he or she acted as such, towards the total flight time required for a higher grade pilot licence: Provided that such pilot is the holder of the appropriate class, type and group type rating"
AND for aircraft requiring a co-pilot
"A commercial pilot, when acting as co-pilot in an aircraft normally required to be operated with a co-pilot, shall be entitled to be credited with not more than 50 per cent of the total flight time during which he or she acted as such, towards the total flight time required for a higher grade pilot licence"
A lot of guys also log full dual hours if the commander is instructor rated on the aircraft, and gives you a bit of instruction during the flight.