So, a UK-EU agreement could well be based on Article 217 TFEU... and so be an association agreement

A small note on the history of Article 217... its provisions were originally drafted in 1957 very much with the UK in mind... and with UK officials trying to influence the wording

UK officials even prepared some draft text to be shared with Paul-Henri Spaak who was chairing the negotiations on the Treaty of Rome.

And, being fed copies of the documents from the negotiations by the Belgians, had the opportunity to reflect on the draft text (then contained in Article 238) and at least consider getting the text amended.

Not that they managed. That said, as one official noted: "‘I think we just have to live with this provision of the Customs Union Treaty as with others which are more objectionable, and this particular one is not really too bad"

More here via @UKandEU: https://ukandeu.ac.uk/partner-reports/a-uk-eu-association-agreement-making-use-of-treaty-provisions-from-1957-originally-drafted-for-the-uk/