And we don't have the systems yet,
Very few (so few I won't even try and research it) countries would be capable of delivering totally bespoke (because Brexit is unique) working systems within 5 years, let alone the one year (well, one year, 4 weeks) till A50 is triggered. And that would be before you factor in the (growing) lack of expertise, and HMRCs IR35 bloodbath.
It really would be easier to put the first Briton on the moon. That's not hyperbole, but a simple fact. It's been done before. The pitfalls are known. The technology and expertise are available, and a 2 years timescale pretty realistic.
I'm more likely to get behind a UK moon mission than Brexit, Boris.
Reminds me of the old Project Manager mantra - "you can have it on time, on budget & meeting expectations"
I first learned that as a software engineer ...
Works,
On time,
in budget
pick two.