I'm suspecting* that Plusnet's new billing system is designed to displace the VAT payments owed to HMRC to a future date by making the VAT appear as Cash Accounting to the customer, but Accrual Accounting to HMRC. That way VAT is only due when Plusnet decides to raise an Invoice rather than when the customer receives a bill.
With Cash Accounting, HMRC is owed VAT when you receive payment from a customer.
With Accrual Accounting, HMRC is owed VAT when an invoice is presented to the customer. When customers have credit accounts, you have to pay HMRC the VAT up front on the sale, then wait for the customer to pay according to the terms of your credit agreement, i.e. 30 days, 60 days etc. If the Customer doesn't pay, then you have to make a claim for the VAT back from HMRC.
I suspect that the bills being presented to customers are not recorded internally or accounted for as invoices, but requests for payment. So no VAT is due until Plusnet decide when is a good time to raise an invoice.
I have no idea if my suspicions are correct, but I received an email from Plusnet to tell me that the new bills I could download on the portal were no longer VAT invoices (they were previously). The new bills have "This is not a VAT invoice" plastered all over them. This is the only explanation I can come up with as to why it matters. VAT payments can be deferred.
* All of this is pure speculation and no one should infer that Plusnet are doing anything illegal.