Please try to understand, the only thing the employer has to do to be legal is to ensure the staff have the ability to take a 20 minute break in the course of, and not at the start or finish of, their shift.
If the employer finds that leads to problems between different contract holders, in that some are paid for a 20 mine break and some are not, or the employer cannot accept the reduced profit margin that results, the employer needs to discuss the situation with the affected staff and find a way to deal with those problems. Imposing a change to custom and practice without consultation is not the way to go about it.
by the sound of it the fact the employer was trying to do that is what triggered the initial post.
so wanting the staff to come in earlier or finish later in order to have that break in their day. that is a viable solution is it not/
and when the employee refuses to agree to change, do they carry on with what is a risk to the employer/