Factual error: When they are driving back home from the holiday cottage you can clearly see an M25 sign in the background. The M25 didn't exist in the 60s.
Factual error: The roadsigns on the motorway are quite contemporary (1980's), the signs used in the 1960's would have looked considerably different. To travel to Penrith, Withnail & I would have had to leave London via the A1, and not the M25 motorway, which wasn't constructed until many years later.