Filming in Yorkshire
They did film some exteriors in Cornwall, but did not film at the real Jamaica Inn. In its place they chose an abandoned barn near Barnsley. (Sounds like funding had a lot to do with that decision.)
http://tinyurl.com/m7pofpm
http://www.jamaicainn.co.uk/
The real Jamaica Inn would hardly have been suitable for filming. It's also very close to a very busy main road.
I think they would have moved those picnic tables. Your suggestion about the road would be an important consideration, though. That, and the money they got to film in Yorkshire.
Mr Browning [Kit, son of dame Daphne] said the decision to base the production in the North had been taken because of a grant – said to be £500,000 – from Screen Yorkshire.share