Just IMHO - you're insane if you don't use push-fit
(I think push-fit == shark-bite)

Push fit is (almost) impossible to connect wrongly.

Anything involving a spanner/thread is almost impossible to connect correctly in tight spaces, like, say, in a retro-fit...


FWIW I do know where you're coming from - it's called old-age and conservatism - I was sceptical too. It also kills 'the trade' so they hate it and diss it whenever possible. (They're also a pretty conservative/dumb lot too).

Nowadays, would you use coaxial cable to wire up an ethernet network? Of course not. Materials have moved on - don't fall prey to the FUD!


What else?
4 elements to plumbing:
* waste (not relevant here)
* hot water supply
* cold water supply
* heating flow/return

Plan each separately.

You'll be partially constrained by existing layouts but you will have some opportunities.

Don't forget that you can easily join plastic to copper so you can replace sections in phases without needing to replace an entire system at once. This can drag out so make sure the brand you select has longevity.

Cold water:
Do you need to soften? Filter? Do you use a header tank - do you need to? If you soften then you need a potable feed to the kitchen (and bathroom sink if possible) - showers, appliances should be soft.Toilet feeds should be untreated to avoid wasting salt. Do you want to use reclaimed water for the toilets?

Hot water:
Less need to insulate with PEX but...
Do you want/need a pumped circuit so the hot water is instantly available anywhere in the house (very nice!)
Do you want mains pressure hot water?
Any desire for heat reclamation?
What about thermal stores?
What hot-water heating system do you use - is it worth changing it?

Heating
I assume radiators.You'll need to check the risers and probably want to use copper/chrome there for appearance.
Check the routing for heating - you may want to zone it and use switchable manifolds - they're not that expensive and it's really nice to have each zone heated independently. No point in heating the bedrooms whilst you're in the lounge/study all day.

If you want you can even automate the zone switching...(later!)

Lots to consider - I suggest you dismiss most of it