low temperature central heating

I got a Worcester Greenstar 24Ri boiler with my house and I'm looking into making it as efficient as possible before taking the plunge at some later date to get a smarter boiler like a two pipe Intergas HRE 18 SB with weather comp and PDHW. But short term, I want to resize my radiators upstairs...

I've installed a wet underfloor heating system over my ground floor subfloor and have just been told by a supplier that I can no longer buy the stiff boards that would have gone under the finish layer. My builder doesn't have experience with UFH so I'm asking for any recommendations for what...