How did you get 605 out of a tank..That's 3 tanks for me...!!
It was on my way home from Nürburgring (I'm not from UK), so we were driving 10 hours straight, highway, with a little detour to see Hockenheim ring (half an hour of city driving)
Actual numbers are 974km, 7.15l/100km, which is ~600miles, ~40mpg
I reckon you can get 650 or even 700 miles (but it would be quite boring), without a detour, without luggage and because there was still some fuel in the tank in the end. I filled up 69,67l, so if full capacity is 80l, there were ~10 litres still left in the tank.
These calculations about total fuel capacity are usually a bit tricky, so I wouldn't bet on the exact numbers..