Theghostbus
1978/9 timetable shows a journey on service 083, daily 1545 ex Victoria, Ipswich arrive 1915 depart 1930, Woodbridge 1950, Wickham Market 2000, Great Yarmouth arrive 2145. This journey is listed as one of those running on 26.12.78
Quite possible that the journey was re-timed a bit from one year to the next.
No indication whose coach it was - it's just listed as 'East Anglian Express' - from the passenger perspective, it was one service even if some coaches were owned by different companies. It may have been that GG always ran certain journeys, or they may have done different coach diagrams in different weeks to balance an agreed share of mileage.
Grey Green (and Norfolk Motor Services) certainly ran some journeys on NX 083 in 1980 - more and picture
here
It's quite possible that duplicate coaches were out on 27.12 - The 1978/9 timetable shows some NX services to / from holiday camps running on 23 and 27 December 1978, and from what I remember, the habit of taking the whole Christmas / New Year period off work was less common in the 70s than it is now, so (Saturday) 27 Dec could have been a busy day with people going home after Christmas. Sometimes, depending on how many passengers are / were going to what points, drivers of duplicate coaches are told to skip some intermediate stops, so will arrive earlier than the scheduled time (some passengers then moan about waiting for an arranged lift / connecting local bus - you can't please all the people all of the time and you can't please some of the people any of the time)
I agree with John C on SCT61 that it's most unlikely for a coach driver to do something 'unofficial' over any sort of distance - the greater the distance, the greater chance of someone noticing it, and the deeper the shit if the coach breaks down / has an accident.
The other thought that had occurred to me is that GG may have run coaches for military personnel going on / off leave, and there could have been a coach to / from London for the holiday. The network of 'forces leave' services operated from the army / RAF camps around Salisbury is fairly well documented (Silver Star then Wilts & Dorset included basic details in their public timetables) - I don't know if there were similar in East Anglia.