Employer keeps adding breaks when I didn't take a break (UK)

I'm currently working a part time job that has me walking all day.

I understand that when I work over 4 and a half hours they have to give me a break even if I didn't actually take it. But I've had a few 4 hour shifts. I don't take a break on those days. But they keep adding in 15 minute breaks!

Are they allowed to do this?

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