So your holiday is pro-rata..
If you work 5 days a week you are legally entitled to 28 days holiday.
you work 3 days a week, so you work 60% of a 5 day week.
60% of 28 is 16.8days.
Then it comes down to how many hours a day you work. all answers above were based on you doing an 8 hour day. where your 0.8 of a day would be 6hours 24 minutes, which would, strictly speaking, mean you'd have to go into work for 1 hour 36minutes and then could take the rest of the day off as holiday.
Although, as I said earlier, if your employer was going to enforce the 0.8 of a day and not just make it up to a full day I personally would try and arrange with them to work an extra 1hour 36minutes on another day so you could have 17 days holiday in total.