There are no specs because the UK's trade agreement hasn't been negotiated with the EU yet
And there won't be any specs till March 2019. So hopefully there'll also be a transitional period of at least two years with no deviation from EU law or the UK will be screwed.
Or maybe the government is already doing speculative execution of possible solutions and the nearest one will be chosen in 2019 and then agiled into the final system/a smoking wreck (you choose). Expensive, though.
Message to MPs: IT is not magic, you do have to tell computers what to do first.