Rest breaks at work
A worker is entitled to an uninterrupted break of 20 minutes when daily working time is more than six hours. It should be a break in working time and should not be taken either at the start, or at the end, of a working day.
As your working day is 6 hours, you are not actually entitled to a break so your employer is within their rights to ask you to make the time up elsewhere.
Of course, this would then put yur working day on a Monday to over 6 hours and you would be entitled to a break as above. It doesn't have to be a paid break though.