WTD and agency part time work

I’ve just signed with an agency and hopefully will get full time work through them eventually but in the meantime I expect I might be doing the odd day or two here and there while they get to know me.

If I were to work for 2 days in a given week, should I be adding 8 hours onto my weekly working time for each of the 3 days I don’t work during that week.
I don’t see why I should because they’re not holidays or sick leave, they’re just days where work isn’t available and I wouldn’t be getting paid for them either.

Please advise.

Short answer “No”.

The long answer. There isn’t one.

Only paid holidays and days off through sickness need to be calculated, although it seems only the minimum holiday entitlement of 20 days need be counted and, since when has being pregnant been an ‘illness’.