If when you leave your current employer, they refuse to pay you your holiday pay, there are certain steps you can take with them, as it is your legal right to receive holiday pay after leaving the company, under the Working Time Regulations 1998:
Section 14, Subsection 2:
(2) Where the proportion of leave taken by the worker is less than the proportion of the leave year which has expired, his employer shall make him a payment in lieu of leave in accordance with paragraph (3).
(3) The payment due under paragraph (2) shall be -
If the company fails to pay you, you will need to take certain steps to ensure that you have the right to appeal, starting off with addressing a letter to the Human Resources manager, titling the letter Formal Grievance, and alowing them 28 days to meet with you, and pay you.
If they fail to pay you, or meet with you (as is laid out from the Statutory Dispute Resolution (also known as the law)), then you can proceed directly to an Employment Tribunal (or call them on 08457 959 775), which will not only be costly for the company (it is a free service for employees to use), but it is also public (bad press tends to be one thing companys dont particularly want).
If you need more information, I suggest contacting ACAS, or an employment solicitor.