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.
Chosen answer: I'm not sure they are friends, they were just thrown together by circumstance. Withnail treats Marwood badly throughout the film. When Marwood gets his role, he smartens himself up and can't wait to get out of there, not even bothering to move his stuff out, saying his dad will do it. I think that they both realise that it's the end of their relationship, and Marwood just wants to get it over with.
Madstunts