Hi GenderCriticalTrumpets!
I applaud you wanting to help your child learn good life lessons before he flies the nest, but I find the way you are going about it a bit counterproductive.
You have obviously raised him to be ambitious, hardworking and focused. He is aiming for uni, works a lot of extra hours on top of studies to make that amount of money and is saving for his future uni costs.
At this point, he will be under increasing pressure with studies since exams will be on soon (if A Levels or the like), so personally, I would prioritise supporting him through this time to ensure his best chance of success. Lots of life lessons and independence skills should have already been taught by now, including budgeting, cooking and housework. If they have not, I would prioritise that over causing family upset over £80 a month.
If the £80/month is the difference between sink and swim for your household, then I would explain that to him backed up by the numbers (as suggested by other PP too). I would probably explain what extras (mobile etc) I couldn't afford to pay for any more, and see if that would offset the £80. I would also reduce time spent cleaning/doing laundry/cooking for him and use it to earn money to help the family budget (unless you have a health condition/disability/other caring responsibilities that makes that impossible).
I can understand you wanting to instill financial responsibility in him, but the sudden demand might have felt like a smash and grab to him.
Hopefully you can try to mend this rift with your son by communicating the whys of your £80/month charge, so that he understands why it is necessary (family budget issues & improving his life skills). If it is not financially necessary, I think your timing is really off considering exam term will be upon him soon, and family support can make a lot of difference at this time. I assume that one life lesson you do not want to teach him is that it's ok for a parent to put the boot in when child is working hard and trying his best.