I'm sorry you don't agree, but I can assure you that is what they are. I know as I introduced them nationally for the national company I used to work for before retiring, and, yes that did include multiple councils.
The difference between an FDD and a VDD is that with an FDD the amount will always remain the same until you are notified in advance of the change, but this normally happens once or twice a year. With a VDD the amount will always remain the same until you are notified in advance of the change, but this can change every month.
No matter what the usage may be, until there is an invoice there is no liability for any VAT, nor can any determination be made on assumed usage be allowed under HMRC VAT regulations.
As for reclaiming VAT, there should not be an issue as it can only be based on the VAT paid, which is specified either on a monthly Invoice or any invoice covering any other period of time. If any person or organisation is basing it on usage, they will ultimately fall foul of VAT regulations, especially if there is a change in VAT rates
FYI I was a CFO of a rather well known service provider.