That sounds illegal to me...
The entitlement to paid annual leave, including the right to compensation payments for untaken leave when you leave your job, begins on the first day of employment.
However, the employer can optionally use an accrual system whereby during the first year of employment the proportion of the leave which may actually be taken (with the employer's agreement) builds up over the year. The amount of leave which may be taken builds up monthly in advance at the rate of one-twelfth of the annual entitlement each month.
Where this calculation does not result in an exact number of days, the amount of leave which may be taken is rounded up to the next half day. Any rounded-up element is deducted from the leave remaining.
At the end of a period of employment a worker will be able to claim for payment in lieu for any leave outstanding, calculated on a pro rata basis from the first day of the leave year or employment to the last day of employment, irrespective of how long that period may be in the current leave year. In this instance, leave is not rounded up to the nearest half day, but is paid on the actual amount due. For example, if a worker had accrued 2.66 days, then they would be paid for 2.66 days and not three days.
http://www.dti.gov.uk/employment/employment-le gislation/employment-guidance/page28979.html#p aid_annual