Calculating annual leave during unpaid leave

An employee hasn't worked for the company very long and has already used up their leave entitlement. They take some additional time off to travel and this is therefore unpaid. Do we include this unpaid leave in calculations for working out how much holiday time they are entitled to?
Example, an employee working 5 days a week for a year would be entitled to 28 days leave, but what is their entitlement if they were absent for another 20 unpaid days on top of their paid allowance - technically they wouldn't have worked a year?
My example is not complicated by pregnancy, sickness or dependants - just wanting to travel!
 

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