There are a lot of factors you need to consider here.
What is the u-val you are trying to achieve for your floor?
What type of Insulation are you proposong to use (PIR or Polystyrene)?
What depth of liquid floor screed do you propose to put in?
How much room do you have from subfloor to finished floor level?
If you have a raft foundation, I imagine you have about 150mm (6 inches) to play around with.
If you use a good quality PIR insulation (eg Kingspan, Xtratherm) you should go with a minimum of 75mm. That leaves 75mm for your screed which should give you sufficient cover for your pipes. If you use a polystyrene board (eg Aeroboard) you will need a minimum of 100mm to achieve the same u-val. that only leaves 50mm for your pipes and screed.
There are differing opinions on whether you should cover your pipework with plastic sheeting. Some of the insulation manufacturers will tell you that the screed will react with the foil coating on the insulation, others are ok with this.
The important thing is to get a tight fit with your insulation, make sure all pipework is properly secured to ensure no pipework floats to the top of the screed after pouring. My advise is to put in polythene sheeting on top of your pipes and bring it up the walls a foot all around and staple to the wall. It can be trimmed off later when the screed dries.
There are many other posts on this topic here on AAM