Caravan owners should get together and do a "Class Action" against Calor. Last year I bought my first caravan and duly paid two deposits for two bottles. Surely taking our money and not being able to supply a refill is a Breach of Contract, multified thousands of times with Calor Gas customers being unable to get refills.
In the UK the nearest we can get to "Class Action" is "Group Litigation"
Wiki Pedia tells us
"The claims are brought as a group, usually with
at least ten claimants and often using the same lawyers. All claimants wishing to join the group litigation must apply to be entered on the group register (i.e. they must "opt in") by a date specified by the court."
so it's not a simple process.
Over recent years there have been several new bottled gas suppliers expanding nationwide network of distributors, so business pressures will cause all suppliers to review and change the products or services they offer. In many cases their gas is cheaper than Calor.
I suspect that if you look at your Calor Cylinder Rental agreement it will allow Calor to change its business model and withdraw some or all of its products from its rental scheme.
If you have only recently taken out a rental agreement you should be able to recover some of your rental charges on a scale set out in the agreement.