You said you made sure that you picked a car that would do the journey without recharging
Well it seems you didnt do any research at all imo
Basic research tells you that wanting a Megan to drive a regular 240 mile mostly motorway journey without wanting to recharge aint gonna happen.
Cant understand why you had thought that would be the case and why not as others suggest stop and chuck a small charge in
Your expectations from this car are far from reasonable imo when considering this particular regular motorway journey
Reading your above replies imo It seems a petrol/diesel would have been the best way for you to have gone.
I suspect they will allow you to give it back as its your first time but suspect because the fault is yours it may well end up costing you £250 to do so.
Seems imo a tiny bit of homework may well have saved you all this hassle
Hope it gets quickly sorted for you