Subway complaint: Short changed

I went in and purchased a bottle of water and handed over 2 x £1.00 coins,was told it £1.79 which I thought extortionate. The salesman then handed me back 20p and said we don’t have any 1p
I said I am owed 1p to which again he said,we don’t have.
I said,if I was a penny short,would you give me the water for £1.78 and he said no. My point exactly,so wanted my change,this was not forthcoming. I have no doubt he or a colleague had a penny to.give me.
Regardless of the amount this should not have happened. I had no.choice but to leave as I had a train to catch and I was not getting any resolution from this establishment.
I would like this resolved in a dignified manner. As I pointed out,1p or any amount,it was my change and should of been given to me.
This is not a trivial matter,so shoukd be treated as any complaint.
The main issue was my girlfriend is pregnant and I didn’t want to stand there arguing,if she was not with me,it would not have been left as it was.
I wait in anticipation for your reply
Best Wishes
Steve