If you have a contract for 30 hours, but in the last 6 months only worked 25. The union can collect the money for the 5 hours every month via court, do you understand?
If you worked 25 hours you’re paid just for those 25 h per month/week not 30 as the contract says. Alongside this if the contract says 30h of work per week/month and you’re not doing 30h per week/month the employer can end your contract. Now it depends what kind of job is and what terms the company has! In general most of the companies will end your contract. Because obviously no company in the uk will pay 30 h if you do just 25h ,court stuff won’t be necessary.