How the limit works
If you're studying at degree level on a Student visa, you can usually work up to 20 hours a week during term time. Below degree level the limit is usually 10 hours. Your limit is printed on your visa or BRP/eVisa — always check yours.
A week means Monday to Sunday, and the limit applies to all your jobs added together. It's a weekly limit, not an average, so working 25 hours one week and 15 the next is still a breach.
Outside term time
You can usually work full time during your university's official vacations. Check the dates with your university, because they're the ones that count.