They are likely to respond to your Notices with scary-looking pseudo-legal letters in which they try to channel you into their bogus appeals process.
The usual trick they employ is to offer you a discount for early payment, like a mugger who doesn't beat you up quite as much if you hand over your wallet without protest.
Words in block capitals like 'LETTER BEFORE COURT ACTION' are designed to trigger a fear response in you and scare you into paying.
It is really just extortion, but the Government are failing in their duty to protect us, so we must find lawful ways to protect ourselves.
By sending them BoEVAT Remedy Notice One, you have agreed to pay as long as they issue you with a proper invoice. This is called 'conditional acceptance' and it removes any controversy, thereby obviating the need for a court to intervene and adjudicate.
They have no controversy to take you to court with, so all they can achieve is to expose their own failure to comply with the Bills of Exchange Act 1882.
Just send BoEVAT Remedy Notice Two in response to anything other than a proper invoice.