With a few exceptions, full-time workers are allowed 28 days leave by statute. They must be allowed the opportunity to take this leave.
There is no obligation for a company to allow the employees to dictate when they take their leave. Ie, leave requests can be refused and they can be mandated (“you’re off on the 15th”) so long as you get the chance to have time off at some point.
The Working Time Directive, as recently amended for coronavirus circumstances, allows you to carry forward up to ten days for up to two years. A company cannot legally just take these away from you, it is a statutory requirement, however as per the previous paragraph they can force you to take them.
Government guidance for giving notice states twice the leave time plus one day. So if you wanted two days’ leave, you’d give five day notice. In practice though your employment contract trumps this. If an employer wants to cancel booked leave then they have to give plus one day, so for example five days’ leave would require six days’ notice to cancel.