I echo what others have said - its unlikely local wedding firms will keep hold of a car just for you; but to make the product more palatable to couples its a great idea to have a backup plan.
Just had a quick look at some car leasing places, and something respectable like a Mercedes E Class saloon is coming out around £350 + vat per month on a 24 month deal.
Or, if you got in touch with a local car hire company you will find a lot of them do deal in higher end cars (as they have agreements to provide cars on behalf of the manufacturer in the event of a breakdown).
It might be possible to do a deal with a local branch whereby they keep something nice available for you to hire in the event of bad Wx in return for a retainer.
I know for a fact that Enterprise used to keep a number of reasonable spec Mercedes on fleet as part of their deal for Mercs Breakdown programme (I know, because I used to have one, and I spent a lot of time in an Enterprise car!!). Perhaps speak to a local branch manager?
Md600 is also correct about the legality of private hire etc; although I am sure a good solicitor could draft up T&C's which stated that the use of the "backup car" was free as a good will gesture but the deposit would be kept in the event of bad Wx.
Hope this helps.
Out of interest, what machine are you thinking of using, and whereabouts are you thinking of operating?