Rex
Certainly very few long haul pilots can claim to have had 8 hours rest immediately prior to flying home. For example, on West coast USA trips, most people wake at about 3am or 4am local time, before reporting for the flight home somewhere between 3pm and 8pm local time. If they are lucky, they may have achieved 2 or 3 hours of sleep just prior to their wake up call.
However, I believe the rules require fully rested and acclimatised at time of report at base. To argue in court that you would not have achieved that downroute and so it is fine to do it before report at base is probably not winnable, because the rules are designed to take account of the lower likelihood of decent rest whilst in the different time zone. BA are probably cracking down on this because they are slowly recognising faitigue is becoming a serious issue, and they want to make sure they are fully covered in that court room should that fatigue related incident occur.