Employees who are paid monthly do not get 'holiday-pay' paid in advance, nor do employees who get paid every 28 days (13 paydays per year)
Weekly paid employees will usually get paid holiday pay paid in advance at their basic rate for the week(s) concerned.
There is a narrow window to take last year's unspent leave in the current leave year, usually by prior arrangement. I have never heard of unspent leave accumulating over several years. Space cadets, time-travellers and those, like Robinson Crusoe, marooned on tropical islands may be exceptions.