If they do it's not a break!
The clue is in the name, WORKTIME learning.
The gov.uk website says
"It doesn’t count as a rest break if an employer says an employee should go back to work before their break is finished"
So, by expecting you to do something that Royal Mail considers work during your break they are not only breaching work time regulations, they are obviously just trying to make the time available to get more delivery achievable.
If it happens again just walk out,you are entitled to take your break off the premises if you so wish