Europcar return car employees are aware of the issue with the fuel gage (peugeot) not showing full upon refuelling.
What I am surprised about is that upon returning the car at LHR you always get a receipt from the clerk and you also get an email with a statement, at least I do! Now if the employee confirmed the car was full upon return, that should be enough for you to challenge, did you get such a receipt or email ?
I rent regularly at lhr and I know the station manager well as I had other issues which he resolved very promptly, if that helps...