The Court seems to have in focus the fact that there has been no clear indication prior to the EU Referendum of the anticipated constitutional exit route in any detail. It may be that the Court will hold that this may be unfortunate but is a matter for the Parliament not the courts. However the scope of the Prerogative is justiciable so this legal aspect will certainly be addressed.
Lord Reed sought to clarify the debate asking Mr Eadie if the view was that purely domestic legislation (such as the Dangerous Dogs Act which seems to have become the default example to illustrate the point) could not be affected by the Prerogative. (The Prerogative not being applicable to purely domestic legislation but rather international treaty). That would contrast with the position if EU law had implemented a Dangerous Dogs Directive as Lord Sumption and Lady Hale noted. In that (Directive) instance the Prerogative could, argued, James Eadie on behalf of the Government’s stance in favour of using the Prerogative, properly be used - as the matter would then stand on the international plane where the use of the Prerogative is accepted. On this view, the European Community Act 1972 (the 1972 Act) could be viewed as transitory, a conduit in place only as long as UK membership subsists. The suggestion, in favour of using the Prerogative, is that the Executive exercises control over the Treaty obligations which pass through the conduit that is the 1972 Act.
An ingenious argument but will it convince the Court? The justices’ probing questions bear no discernible relation to their ultimate considered opinions but the broad flavor of discussion so far does seem to point to a significant majority, at least, of the Eleven holding against Mr. Eadie on the question as to whether or not the Prerogative power was the appropriate constitutional mechanism in the circumstances.
The HS2 case shows that the Courts will uphold central constitutional principles and parliamentary sovereignty as against any perceived incursion from EU law. It would seem in conformity with that approach to uphold parliamentary sovereignty as against the Executive.
It all likely boils down to the distinction between what is law (and justiciable), what is a matter for politics (and thereby non-justiciable) and the respective role of the Court in relation to the Executive and Legislature.