I too remember PanAm DC8s at LHR but it was long time ago and I thought they didnt have 'fanjet' engines as they called the early turbofans. This would have meant the PanAm versions were DC8 -30s so theywere DC8 30s
When National came along I think they were all DC8-50s ( The 30s might have struggled to make LHR-MIA) with their distinctive Yellow and Orange colours and B17 style girls names under the flight deck windows as part of their I 'm Mandy Fly me campaign.
May be wrong of course it was long while ago although LHR had great variety in those days