Assume nothing (a mantra I live by as assumptions are the mother of all Foul ups)!
Can you post a photo of your UFH manifold? I posted a lik to a blending version on this thread Cost of running a Nibe ASHP , controlled by SMO20 for DHW only - #37 by borpin. basically it limits the temp to the UFH and draws heat from a store as and when needed.
So the WC is more impacting the ASHP than the UFH. It does do a bit of it’s cold outside so need to come on earlier, but I suspect that is about it.
UFH works best at a steady state (IMHO), so I never let mine get really cold in winter and I then only need to trickle feed heat into it (low deltaT) and maintain about 28°C flow temp. Most ASP systems tend to connect the UFH directly to the HP. Personally I think this is wrong and a common source of the HP cycling.