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Oh wow you've just taken me back to a camping trip at the age of 18 in my mother's Vauxhall Cavalier in 1996. The road was steep enough that we decided to sit one of our group on the roof of the car (with his feet dangling through the sunroof) so he could shout out as each turn crested whether it was to the right or left.
From recollection it is a very steep, but not especially narrow, road. Regen coming down should help you avoid overheating the brakes.
To be honest it hadn't really occurred to me that the Model S would be too wide for any road in the UK. It's not as wide as a Range Rover and they go everywhere!
Also you can always use google streetview to take a look
Your post made me learn of Hardknott, Wrynose, Kirkstone and Rosedale Chimney. Hmmm....how to get the two of us and the Model S Over There to have some fun....