Silsoe, you sir, seem to have lost the plot.
Firstly we are referring to the D&C EC145 NOT your EC135!
It looks like in the case of the EC145 an extra passenger has very little (just a few Kgs) effect on fuel consumption & endurance. So an extra tasking may not be affected due to a lack of fuel as you can take off with full fuel.
Secondly if were going to drag your EC135 into it then do you normally sit your EC135 outside with full fuel or 400kg?
If you want more fuel to start with then surely you turn the passenger down! If there was a requirement to have the passenger in the first place then to be quite honest it's tough cheese on the pursuit but I guess you could discuss and generate options with your fellow crew and perhaps a plan could be formed for another NPAS unit to get airborne and assist? Hawarden? Manchester?
Otherwise I guess sometimes the bad guys risk getting away! But then ground officers can always make enquiries afterwards!
Is there an SOP of not having an aircraft sat with full fuel in case a passenger needs to be flown? I've seen pictures of dog handlers flying before now.
H500 - it might seem bizarrely professional but the Police may well have had there reasons to be on the Scilly Islands which could include a requirement to conduct up to date landing site surveys for night landings on a NEW part of their patch? And of course whilst you're there and you have a spare seat why not utilise it?