Your statutory minimum entitlement is calculated as 5.6 times the number of days you work per week. So you're entitled to 22.4 days paid holiday (where, of course, you'll be paid for 4 hours on each day).
That allowance includes all 'enforced' holidays, where the business is closed (such as public holidays, if the business doesn't operate on those days).
The employer has discretion as to how to deal with the awkward 0.4 of a day. You could simply be allowed to go home 1.6 hours (1 hour 36 minutes) on a normal working day, while still receiving full pay for that day. It's more likely however that an employer would actually give you 22 days holiday and just pay you for an extra 1.6 hour's work.
Of course your employer is free to offer more holiday within your contract of employment but that's entirely a matter between you and your employer, and not subject to statute law. However your employer should (by law) have provided you with a written 'statement of employment particulars' within two months of you starting work, which should indicate your holiday entitlement.
Chris