Did you do 4 hours followed consecutively by 1.5 hours ? If you did not do them consecutively and had a break between them, in my opinion, RM should not have deducted the break.
If you did them consecutively you did a total of 5.5 hours which will automatically get a 15 minute break - without being paid. Note: in the handbook, a copy of which is on this forum, you get: a 15 minute break for, I think, 5 hours to 6 hours 59 minutes - which is illegal; for 7 hours to 11 hours 59 you get 30 minutes break and 12+ hours you get 1 hour break but check the handbook . The legal minimum is 20 minutes for 6 hours work.
AWR is Agency Worker Regulations, there are a number of threads on this forum about this with links to government documentation about it from the BIS department. Suggest you check them out as Angard will not tell you about it. -