The UFH manifold in my old property was speedfit pushfit too so they must expected pretty much continuous use there.

Indeed Speedfit appear to be promoting the UFH applications, so it is a bit of a mystery as to why a heat pump would not be supported. I would think that matters is the flow of water and the temperature rather than the thing which warms up the water.

After 10 years no problems.

I felt that the Speedfit system's guarantee had a certain value to it (they being confident that the pipe will last), otherwise one would use other brands of polybutylene (or Pert/Al/Pert) pipe at half the price.

I installed my first heat pump using JP Speedfit fitting entirely. After 10 years no problems. I didnt run it continuously though as the property was super insulated. Just fitted a 5kW samsung in my new property using the existing radiator pipework but used speedfit elbows on the radiator tails but I did use Hep2o inserts and pipes as they are thinner and metal.

The UFH manifold in my old property was speedfit pushfit too so they must expected pretty much continuous use there.

If you have any awkward joints which would be a bitch to get to you could always put a compression joint on those.

From my experience I have had no issues using plastic pipe and fittings with a heat pump. My flow rate is fine as is my COP. Perhaps there might be more of an issue if using a larger sized heat pump?

The helpdesk stated that the guarantee applies where it will not be constantly running, which seems to imply that a finely tuned heat pump (one operating at reasonable flow for half the year) is out of guarantee. I was given the contact details of second line support but they've not got back to me on this issue.