Apple refuse to refund me?

Hi there so while going over my bank statements I noticed that my YouTube premium subscription had been coming out of my account twice a month for a year costing £31.98 in total instead of the £15.99. I called apple and was only offered a partial refund of £47. Out of the £191.88 total I had been over billed. I only have one gmail address registered to the account and told Apple I could prove this to show their was not possible way for me to be using this second subscription I know nothing about. I was told “well to be honest you should have noticed sooner so we can’t offer you anymore of a refund” I thought that wasn’t the usual response Apple would give to asked to speak to a higher up and he was just as unsympathetic to the issue. Felt like their view was “we no it was a mistake and you couldn’t possibly have used the addition membership but we have our money now so go away” I asked to issue a complaint and was told “someone may or may not get back to you” which didn’t fill me with much confidence. Don’t know what to do next?