Medical experts usually say the average person can live for 4 to 6 weeks without food. The amount of time someone can go without water is much shorter, roughly a week (or less).
Your employer can force you to take leave at a particular time as long as they give you at least two weeks' notice. However, it is common for employer and employee to come to an agreement about the timing of annual leave. If you put in a request for leave, your employer has to have a good reason to refuse it.
You must give your employer advance notice that you want to take holiday. This notice should be at least twice as long as the amount of holiday you want to take. For example, you should give two weeks' notice for one week's holiday. Your employer can refuse permission for your holiday as long as they give you notice which is at least as long as the holiday requested. So to refuse a request for a week's leave, they would have to tell you a week in advance.