UK Clients/Brexit/VAT/Customs

I am an auto-entrepreneur (BNC professional liberale) and I provide services usually to clients in the UK. I reached the threshold for VAT last year so I went through the process to request a FR VAT number and I declare VAT every month (Livraisons inta-communautaires ventes B to B) as well as to customs (douane DES) every time I issue an invoice.

There was some confusion from my side on VAT declaration. I assumed that it was based on every tax year. So I did not declare VAT when I invoiced a UK customer in January and included the usual “TVA non applicable – art. 259-1 du CGI” in the invoice.
I invoiced my client very early in January and an accountant told me to continue as before Brexit as there was not yet any guidance.

I cleared up the issue with the French tax office that I do need to declare VAT as I met the threshold last year and if I am under the threshold this year then I need to email them at the end of the year to let them know.
This means that I have invoiced my UK client incorrectly and now need to deal with customs and how to invoice a UK customer.

Please may you advise me on the following:
1) What VAT information do I need to include on my invoices to UK clients?
2) How do I update and reissue on the old invoice from January?
3) Where do I declare to customs? The usual douane DES does not work for GB tax numbers anymore.

Many thanks in advance.