Her Majesty’s UK Government (“HMG”) introduced UKIMB 2019-21to “clarify” some terms in the Agreement to withdraw the UK from membership of the EU. Here we focus mostly on ‘good faith’.
“The Union and the United Kingdom shall, in full mutual respect and good faith, assist each other in carrying out tasks which flow from this Agreement” (the Withdrawal Agreement or WA) 
Theresa May made famous the phrase ‘Brexit means Brexit’  but what did she mean? Was it:
a) Britain would exit the EU and become a sovereign state; or
b) Britain would accept laws and regulations from a foreign authority that is not accountable to its own citizens.
For her and many others Brexit meant b). Boris Johnson had to overturn the Fixed Term Parliament Act by persuading a Remainer majority of MPs that he would not pursue a ‘hard’ Brexit to achieve the other interpretation, Brexit means a). That meant he couldn’t scrap most of what she had wasted her Premiership agreeing.
UK sovereignty was asserted in the minor revisions to the WA accepted by the EU and Parliament . However, the sub-clauses take away our protection, giving the EU extraordinary powers, if it chooses to abuse them. The default setting is that all shipments into Northern Ireland are to be deemed “at risk” – so the UK Government will have to collect tariffs. It is an important principle of our constitution that Government and Parliament can decide what the law is.
To accept the EU’s interpretation that it could rule the roost in perpetuity would break an international agreement that the EU agreed it wouldn’t break. The Agreement cannot be accepted or rejected without breaking international agreements, notably the Good Friday or Belfast Agreement, therefore it has to be changed or a safeguard introduced to protect the UK from malicious misuse of the WA provisions.
The Good Friday agreement is also an international treaty. The WA cannot override it and impose a new constitutional regime on the Unionists without the consent of the people of Northern Ireland. The UK internal market bill is a necessary safeguard. It is to be activated only if the EU acts in bad faith.
The Protocol on Northern Ireland  doesn’t do what it says it is supposed to do. Therefore the UKIMB introduces such a safeguard to prevent an international agreement being sabotaged. The whole point of the Protocol was allegedly to avoid exactly this danger. our lawyers have embraced a radical supranational definition of international law, one which is shared with the EU. The confusion leads to errors of interpretation
Some issues were moved from the formal international treaty, the WA, to the Political Declaration (PD) in Johnson’s quick renegotiation of Theresa May’s appalling version. However, the signed WA still says of the PD that both parties will use “their best endeavours, in good faith and in full respect of their respective legal orders…” where “good faith” and “best endeavours” have strong legal implications.
The proposed UKIMB (it is still going through both houses of Parliament) includes safeguards which can be implemented if the EU tries to use its residual powers to interfere with the UK’s internal market .
While much of the ‘debate’ centres on legal matters and we are not lawyers, Martin Howe QC has given a strong interpretation in an extended paper , from which we quote:
“Under the UK’s constitutional law, international treaties in general do not form part of the law. Where an international treaty has made it necessary to make changes to the UK’s internal law in order to comply with it, the general practice has been for Parliament to pass legislation to make any necessary changes to the law.”
“In a provision which was not merely rare but unique, Parliament legislated in section 2(4) of the 1972 Act to give the EC/EU Treaties and subordinate instruments made under them so-called primacy or supremacy over all laws of domestic origin, even over Acts of Parliament.”
“Under Article 4 of the WA, the WA itself and its Protocols including the Northern Ireland Protocol (NIP) are required to be given direct effect under internal UK law, and to be given supremacy over all other UK laws. Further, the EU law and EU law concepts which they continue to apply to the UK must be interpreted in accordance with rulings of the ECJ.”
“…on 31st December 2020, after that date the WA and the NIP will continue to be given direct effect and supremacy as a matter of UK law”.
“As UK law now stands, ministers have no legal power to prevent them having effect as part of UK law, or to stop HMRC or the civil service from applying them or to stop the courts from enforcing them.”
“This wide ranging ‘direct effect’ provision meant that not only the EU Treaties themselves, but numerous subordinate instruments made under them such as EU Regulations, Commission decisions, and ECJ judgments, also directly formed part of the legal rules enforceable in UK courts.”
“It is…regrettable that the doctrines of supremacy and direct effect were carried forward into the Withdrawal Agreement, to continue to be applied to the UK for indefinite and potentially lengthy periods of time….Extraordinary jurisdiction clauses were meekly accepted by Theresa May’s Government under which a court of one party (the ECJ) was given extensive jurisdiction over the supposedly sovereign other treaty party.”
“The Government’s UK Internal Market Bill is needed to maintain the free flow of trade across the nation in the post-Brexit world.”
“Two clauses in the Bill…would allow the government to restrict the so-called ‘direct effect’ of two parts of the Northern Ireland Protocol…. These clauses have the purpose of protecting the basic functioning of the United Kingdom’s internal market as it operates between Great Britain and Northern Ireland in the event that it is not possible to reach an agreement with the EU on the UK’s future relationship with them…There has been little sign of any degree of realisation or acknowledgment of the extremely serious consequences for the United Kingdom generally, and for the people of Northern Ireland in particular, that could automatically ensue if these clauses are not passed into law.”
 The 2019 WA: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/840655/Agreement_on_the_withdrawal_of_the_United_Kingdom_of_Great_Britain_and_Northern_Ireland_from_the_European_Union_and_the_European_Atomic_Energy_Community.pdf
 Martin Howe QC (Director of Briefings for Britain)