I work in a cafe as a weekend job But the pay is not consistent help?

And do u have proof of those 18 hours worked? E.g. a signing in and out register (i had one when i worked in a coffee shop) then take a picture of those hours for proof and if they did not inform you beforehand that they will reduce your rate than you have the right to present proof of your hours worked and demand the remaining pay you were not given.

Did they inform you before reducing your hourly rate? @Deorbiting

Invest in your education man, £80 is chump change. Take that £80 and go buy some revision guides, those will return 100 times on your investment. If you don't need the money right now, focus on your studies.

It not that I need the money he asks me to work then pays me sh it money and he usually calls me with no notice plus my grades are not too bad I have a tutor and all that 3-4 times a week

If you feel he is taking advantage of you, then quit. Like you said, you don't need the money. I'd rather you focus on your education.

Your hourly rate and other details about how much, how, and when you get paid should be in your contract. Do you have a contract with them? You shouldn't have to guess this sort of thing, it's management's responsibility to give you clear information

I would suggest asking your manager if you can have a word, and politely explaining that you seem to be getting paid less per hour than you used to be, and could they explain the pay calculations to you? It might seem scary at first but a reasonable manager should have no problem with that. As the poster above said, if you have some kind of proof of what hours you worked, that helps you support your case, even if it's just that you or one of your parents has a diary and has notes of when they were dropping you off, for instance.

You say you're 15 - whereabouts do you live and when is your birthday? If you are school leaving age, you're entitled to the national minimum wage, which would be £4.20 per hour.
Here is a link about national minimum wage: https://www.gov.uk/national-minimum-wage-rates
Here is a link to work out whether you're classed as school leaving age: https://www.gov.uk/know-when-you-can-leave-school

If your friends have part-time jobs, you could ask them about what the pay is like - if the managers won't pay you what you feel is a fair wage, you could consider looking somewhere else. I know it's like a taboo talking about your pay in the UK, but it's actually really important to know you're being treated fairly!

Good luck, let us know how it goes!

My boss and his manager were there and they confirm the hours

I live In London and my birthday is the end of may
I don’t really have a contract with him since I only worked there a few days and then it became a regular thing

OK, so it sounds like legally you're entitled to the minimum wage from the end of June.

If you want to stay in that job, I'd suggest saying something like "I've been here nearly a year, so I'd like to get the details of my job down in writing/in a contract." Ask to discuss hourly rates. It sounds like this guy is really informal - does he pay you cash in hand?

And do u have proof of those 18 hours worked? E.g. a signing in and out register (i had one when i worked in a coffee shop) then take a picture of those hours for proof and if they did not inform you beforehand that they will reduce your rate than you have the right to present proof of your hours worked and demand the remaining pay you were not given.