It must be totally different in retail, if you start work your holiday entitlement starts to buiold immediatley. I started my new job Oct 1st, with a holiday year 1st april to match 31st, so I get 2.5 weeks paid holiday before end march, i have taken 1,5 already and am taking the final week next week. If i didn;t do that, by 31st march the firm would owe me the 2.5 weeks and i would lose them as they don;t carry them into the next year nor do they pay in lieu.
If the holiday year is january to january and you start in january, you need to take your 4 (or 5) weeks over that 12 months as you will be owed that time by the end of december