Working hours are governed by the Working Time Regulations. Most workers can not be forced to work more than 48 hours per week on average. The average is calculated over a 17 week period. Your employer can not dismiss you for refusing to accept longer working hours - even if your contract requires you to do so.
Workers can increase their working hours by opting out of the 48 hour limit. To opt-out, you need to put your request in writing, sign the letter and send it to your employer.
+ lake transport worker on board a sea going fishing vessel, sea transport worker
+ mobile worker in inland waterways
+ domestic servant working in a private house
+ company directors who can freely choose their working hours.
+ emergency services, police and armed forces
Workers are required to work on Sundays if their contract says so. Sunday working hours can not be forced upon workers unless they have a contract agreement to work Sundays. Where there is no contractual agreement for Sunday work hours, an employer who forces a worker to work Sundays would be in breach of contract.
If you are a shop worker (open to the public) you have special rights. You can opt-out from working on Sundays even if your contract requires Sunday working hours. If you only work on Sundays you can not opt-out.