Holy ****e batman!
1800kg empty mass
400kg of role equipment (generous!)
270kg for 3 crew
90kg for the disputed pax
694kg Full Fuel
= 3254kg
MAUW = 3585kg so one would like to think in the UK on an island near sea level there's a good enough margin for a Class 1 departure at 3254kg.
So then we look at the fuel consumption chart for operation and see that a higher weights at 90kg passenger makes sweet FA difference in the grand scheme of things - a few kilograms at best! And some pilots can waste that much fuel quite easily or even save that much fuel quite easily.
Cabby - The problem here is that SS posts rubbish for the sake of stirring rubbish. It's quite clear once you wipe away the rubbish.