I liked a piece in Computer Weekly that commented on May's speech on Friday and said:

Border IT is however one of the biggest tech challenges when it comes to Brexit. Of the 85 IT systems at the UK border, 30 will need to be replaced or changed, but the government has said it does not expect “all new or updated IT systems to be ready” by the time the UK leaves the EU. HM Revenue and Customs (HMRC) is in the middle of implementing its new customs IT system, however, the department can’t guarantee it will be ready on time, and has been urged to go for a basic version of the system.

So we need a transition then or, to not put too fine a point on it we're ...... well, you know the rest.

It's always worth reminding ourselves of the realities Rees-Mogg ignores. And let's be clear: he can't ignore them. If we are to trade with the EU after March 2019 we have to give them data. If we can't, we won't trade. It really may well be that simple. They are not going to break their law to do so.

In that context another comment is also interesting:

May said she recognised that some of her ideas for a future customs and trading agreement “depend on technology, robust systems to ensure trust and confidence, and goodwill,” but that “they are serious and merit consideration by all sides”.

'When they're available' is, I think, the unwritten subtext. Right now they're not. Living on planet possible says it is the UK who is going to be doing a lot of compromising very soon.