New theory: could he have tried to make his way back to one of the previous pubs (Landsdown Arms, Royal Oak)? Lost his way? Couldn't find the way back?
As an aside, looking at the several cctv clips I found myself puzzled that this was described as a very busy night when everyone and their uncle are out for Xmas drinks/dinners/shopping/socializing etc. To me it looks like the dead of night. If I hadn't been told I would have thought the footage was from 3am or 4am. I expected crowds of people, heavy traffic etc. Instead we have one van passing a couple and someone standing opposite, or three people walking down the sidewalk in a dark empty street ....