I used to employ in my retail business a lot of people ,
I put them on 4 hr contracts with agreement they could do set hours per wk
As over time above the 4 paid at time , I then had them working 16hours a week & more ,
But only had to pay hols entitlement for 4 hours they were contracted ,
I also kept them below the ni threshold as I didn't have to pay ni ,
Had no problems with this , as most kept below certain amount for benefits & wanted this any ways ,
Only then obligated for for 4 hrs contracts , They also had flexibility as well ,