Given the rank hostility experienced from Macron when Britain exited the EU, it would be a sweet revenge to politely decline his offer.
But politicians and pragmatic by nature, it's part of the job description, which is why Monsieur Macron is offering the deal, and why Britain will consider it, and not reject it out of hand.
I fail to see any obvious benefits to such an alliance, and plenty of downsides, but then I am not a politician - they may see something in the arrangement that I can't.
That said, since Mr Johnson swept to power on a fulfilled promise to extricate the UK from the tentacles of Europe, I can't see any political benefit in basically moving the chairs around, and sitting around the table again.