On a nice steady run, in my experience, you’d ikely get 25mpg (if it were a V12 5.3l then you’d be getting around 17mpg).
One thing to be cognisant of interpretation of reading the fuel gauge in your can and how much fuel is in the tank when it reads 1/4 full. Until you’re sure of what you see then don’t trust that 1/4 on the gauge is actually a 1/4 of the tank of fuel – fill up early and carry spare fuel (though you really don’t want to run out as it will suck up crud from the bottom of the tank that you really don’t want distributing throughout the fuel system).
Better safe than sorry on your long trip to Isle of Mann to fill up regularly and then when on home turf to let the fuel levels lower and estabilish for yourself when exactly on the gauge the light comes on and how much there is in the tank at that point).
Hope that helps – have a safe and uneventful, though enjoyable journey