Can an employer reduce your pay if you request to work night shifts only?

Question

This agreement includes 2 weeks of day work per year. This is for the U.K. and is relevent to nursing staff in the NHS.

Answer 1

I guess I certainly can't speak for the UK, but every position I have ever held, the night shift paid an additional premium to the workers.
It's called "shift differential pay."

Answer 2

Slavery in UK. Employees have no rights.