When you leave your job, you should be paid for any holiday you have not been able to take during that holiday year.
However, your employment contract may entitle your employer to demand that you take your unused holiday when working through your notice. Check your written contract terms.
To find out how much holiday pay you should be paid when you leave, calculate your leave entitlement, based on how much of your current leave year will have passed by your last working day (your company may set a standard leave year, which you can find out about in your staff handbook, or you may have an individual leave year, which will run from the date you originally started work with your company). You are entitled to one twelfth of your annual holiday entitlement for every month you have worked of the current leave year. The GOV.UK site has a useful annual leave calculator.
If you have spare entitlement when you leave, your employer should pay you your equivalent daily pay rate for these days.
Be aware though, that if you have taken more than your entitlement for the leave year to date, you may need to pay back your employer for the paid holiday you have taken but not yet earned.