Any unused holiday entitlement that you have built up must be paid to you when you leave your employment – even if you are dismissed!
Note also that if you have taken more holiday than you should have done at the date you leave, your Employer can claim this money back from you. This is often done as a deduction from your final wage or salary, and your Employer needs permission to do this by having a clause in your Contract. If you think your Employer has unfairly made a deduction from your wages take some further advice.