DIY Underfloor Heating

I am in the process of fitting a new bathroom 'from the bottom up' and am interested in incorporating a DIY UFH system which connects to the existing radiator system.
I am laying 12mm flexible piping, but was wondering if anyone had any experience of the necessary flow control valves and return temperature limiters that would be necessary to have a lower flow temperature in this circuit.
Is this something that I can fit to a manifold system so that I can avoid having to pay an arm and a leg for a 'dedicated' system such as the Wirsbo Push 12, which seems to cost about 350 Euros.
Any help / advice would be most appreciated.
Many thanks.
Andrew M near Albi in South West France.
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