The UK signed up to take Northern Ireland out of the UK single market. No one forced it to. It could have done no deal with Europe. But it chose to have a deal. This was part of that deal. The UK chose to split its single market, leaving Northern Ireland in the European market.
The UK now seeks to deny this, saying Northern Ireland must be in the UK single market. That is impossible under the terms of the deal it signed with the EU.
The EU say the deal was written in English to make sure we understood it. We must honour it as it was written. The demand is entirely reasonable.
This all being entirely different from signing up to the euro – say, a newly independent Scotland, or a Britain that returns – but never actually doing it. For to do it would mean we couldn’t use MMT and that would be horrible.
To the P³ keeping your word depends on the utility of doing so……