It seems to me that all of the 1,000 or so engineers who have refused to agree to the change in employment terms will take a case for constructive dismissal before an employment tribunal.
Hopefully a class test case would be considered rather than wasting tribunal time.
Centrica will no doubt argue that the change in terms are minor and so should not lead to successful claims. But Centrica has said that the changes are necessary to protect jobs as it tried to shore up its market position while battling to compete with rivals (and so cannot be considered minor).
If the claims are unsuccessful, the tribunal should indicate what level of salary reduction they consider reasonable, and not considered justification for a constructive dismissal claim.
https://www.dailymail.co.uk/news/article-9470465/Hundreds-British-Gas-engineers-share-fury.html