I had to do that for my garden sprinkler at a house I once lived in.  Got around it via a Fred Karno method by using a large water butt with a submersible pump within attached to the hose to the sprinkler.  A hose from the garden tap running constantly into the water butt. Bit of a faff but it certainly upped the water pressure to the sprinkler.

It would be nice to fit a pump to my garden sprinkler in summer but I suspect it would simply draw air in from the water tank feed or the kitchen tap when used.

Fair enough.  With poor mains pressure the only advantage of a UVC is that it is sealed and so less likely to get general detritus in.

@morgan  Why not?  There’s hydraulic separation between the ASHP flow and the domestic hot water

Coil probably not large enough, although this can be overcome by 'plate loading' ie fitting a phe and pump to replace or supplement the coil.

I debated retaining my vented cylinder but was persuaded to change it for a uvc (by an installer who was quite happy to fit a supplementary phe if I wanted him to).  For me definitely the right decision, no pumps anywhere, balanced, potable hot and cold everywhere, better water flow.  I replaced my pumped shower with a very smart but simple, non thermostatic, mixer (Grohe, about 60 pounds).  The only downside is that the airing cupboard is now barely luke warm because the insulation on the new cylinder is so good, even though I instructed the installer to omit the insulation on the adjoining pipework.  Of course this decision is very circumstance dependent.