I do exactly as SurfOnYourBum said on my Rover 200 to get my 9'2" in. Mine sits in straight (not diagonally) so my gearstick movement isn't restricted. Seatbelt definitely helps!
I've done this many times on Sussex - Devon 250miles drive. I don't think it's unsafe or illegal. The board doesn't inhibit my sight to nearside or rear. My rover does 48-50mpg on that journey (driving 65mph) with boards inside.
Also I've done the same journey with my 9'2" and 6'5" on the roof (detachable Halfords roof bars with pads though) and a passenger with our kits for the weekend stay. Fuel efficiency goes down to around 39-41mpg (similar speed as above). Almost 10mpg difference.
When you do a geeky calculation:
250mile journey at, say 48mpg (board inside), will cost me £31.47 at 132.9p/L price.
250mile journey at, say 39mpg (board on the roof), will cost me £38.73 at 132.9p/L price.
That's £7.26 difference. £14.52 more for the return journey.
It’s up to whoever is paying for the fuel to decide if that’s tolerable or extortionately expensive.
Another option is to leave your longboard at a storage (if surch service exists?) down in the West, maybe?
Or invest on hard roof bars? Anecdotally speaking, it might help reduce the drag compared to soft-racks which I've seen some longboards pointing straight up the sky and creating a parachute effect...
I hope this helps to give you an idea on fuel consumption with/out board on the roof.