My uk employer has underpaid my holiday pay for 6 years how do i claim it back?

Ask your employer. If that doesn't work (and you can demonstrate you were underpaid), contact ACAS, the employment law experts, who handle anything to do with employment queries and problems. They will give you free, impartial and confidential advice. Masses of useful info, plus contact details, on their website: www.acas.org.uk