Continuity mistake: Whenever you get a shot of the car travelling along the motorway or road, the cars going in the same direction as the Jaguar are all true to 1969 - the year the film is set - but the cars travelling in the opposite direction or parked by roads are all contemporary with the time it was made (eg. Ford Sierras).
Continuity mistake: When the car leaves the motorway on the sliproad as Withnail & I first head out into the countryside, there is only one person visible in the front of the car. Withnail said he was going for a sleep, but it would have been too difficult for him to make it into the back of the car whilst it was moving.
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.