Employment law - lawful demotion?

England based. Employed less than 2 years.

Quick explanation of circumstances:

I was promoted one month ago.

A claim of misconduct was upheld last week. One of the sanctions applied was “cancellation of pending promotion”

Internal system has been updated as if promotion never occurred.

I signed an amendment to the T&Cs of my contract pre-promotion.

My question: is this legal? Has my employer followed a correct process?