Its my belief that the security of the royal family comes directly from the Metropolitan Police/Scotland Yard and those that work protecting the royals such as RPOs are paid directly from an allotment of funds granted to them by the government and hence the taxpayer's are footing the bill. An example being that Charles' RPO does not work for Charles. He works for the Met Police/Scotland Yard.
The cost for security for this carriage ride will not raise the amount the taxpayers are paying in their yearly taxes. If the security forces felt that it would have been above and beyond what their funds would allow, they would have advised the Queen or anyone in charge of that carriage ride that it would have put a huge onus on the money they have available to pay for the security on October 12th.
I've heard of no complaints from those that will provide the security.
I've heard no complaints from the town of Windsor itself.
I've heard no complaints from the businesses affected in Windsor.
Only complaints I've heard is from some that pay UK taxes.