I think it's important to stress what Flowery said above, as other people may read this thread:
Contracts can give more rights than the law allows, but the position if the contract is silent on any matter would be the minimum the law gives.
The law on holiday (taken from the Gov.uk site) is:
Employees are entitled to 5.6 weeks holiday, up to a maximum of 28 days.
They have the right to:
get paid for leave
build up (‘accrue’) holiday entitlement during maternity, paternity and adoption leave
build up holiday entitlement while off work sick
choose to take holiday at the same time as sick leave
The general notice period for taking leave is at least twice as long as the amount of leave a worker wants to take (eg 2 days’ notice for 1 day’s leave), unless the contract says something different.
An employer can refuse a leave request but they must give as much notice as the amount of leave requested, eg 2 weeks’ notice if the leave requested was 2 weeks.
Employers can:
tell their staff to take leave, eg bank holidays or Christmas
restrict when leave can be taken, eg at certain busy periods
There may be rules about this in the employment contract or it may be what normally happens in the workplace. The notice period for this is at least twice as long as the leave they want their staff to take.
In the case of the OP, she has been given over 3 weeks notice that she must take 1 week of leave.