In view of the attempted prosecution of Boris I bring to your attention:
Wheeler v Labour Prime Minister (2008) the Divisional Court held that manifesto promises were NOT justiciable

" "Even if we had accepted that the relevant ministerial statements had the effect of a promise ... such a promise would not in our view give rise to a legitimate expectation enforceable in public law, such that the courts could intervene... The subject-matter, nature and context of a promise of this kind place it in the realm of politics, not of the courts."

I find it surprising that action against Boris was not taken at the time in 2016. This late in the day it is a deliberate attempt to marginalise a contestant, the same as the attempted smear of Farage by an electoral commission enquiry inmmediately before voting commenced.
Compare and contrast this with the delay until after the EU elections for the Equality and Human Rights Commission (EHRC) announcing the launch of a full statutory investigation to determine whether The Labour Party has unlawfully discriminated against, harassed or victimised people because they are Jewish.

The only other political party to ever face a formal investigation by the EHRC is the BNP.

Politics is a dirty business!