My opinion but having never hired a MH, is hire in UK local to home
That way they will leave fully packed, knowing exactly what is in, it would be difficult to turn up and find yuo need something critical for the holiday that is not included (I guess reading the small print would avoid that)
the only negatives I can think of are ferry costs will be higher than car and they will be in a RHD MH possibly for the first time abroad
I hired a MH to go to France in June and used Just Go at Flamstead, just off M1. Very helpful people and good choice of vans. However the drive to Folkstone nearly sent us round the bend ! After the paperwork and demo, we chucked our bags in and set off. However it all sounded like lose spanners in a tin bucket even though it was a brand new van. Everything rattled like crazy. A piece of kitchen roll between all the plates and tea towel around the grill pan soon sorted it out.
Oh - check what oven in provided, I was told it had a gas oven but it turned out to only have a gas grill !