I have asked this myself
They will not loan a van
They will only rent you a van, but it's around £70 plus vat a day (I may be wrong)
Personally I think they are missing a trick, if they had a loan van, free of charge I'm sure more would order hot water systems, that as we know take at least two /three days to install.
Logistically, they drive down in the loan van, drop to you and drive yours away, surely it would benefit them as they either have to bring 2 blokes or leave their van parked up at your premises for the three days they are away.
Personally depending on finances and work due, I would probably suck it and take the van rental.
Just my thoughts
Mr B