of research before opting for a Heat Pump, heat calc surveys, where to site HP, pipe routings, where to site DHW, and how to get heat round the house.

I opted to remove the ceiling as I didnt want to disturb upstairs rooms get the floors up, to allow changes to Flow Return pipes when old boiler removed, and upgrading feed pipes to each radiator upstairs to 15mm copper.

Warning for future readers of this post:

PEX is useful but be aware the internal diameter is smaller than same size copper pipe for the elbows, T’s etc as you have to fit those inserts to support the pipe which reduce ID so 22mm PEX has equivalent ID to 15mm CU, to maintain same Flow Rate.

If you cant get the flow rate high enough, you will need bigger emitters!

After my system was commissioned, I had issues with slow heat up times in bedrooms; Octopus suggested bigger radiators. I argued for a higher flow rate and had all the 10mm microbore removed and replaced with 15mm CU.

We split the cost 50/50 as it wasnt their preferred solution so wouldn’t absorb the whole cost

Could I leave some pipes in that space, ready to be used in a year or two? Or some conduit to make it easier to feed pipes through? Or will an installer be able to push pipes in from the end if I can mark the right spot? It's about a 3 meter run, parallel to the joists.

That's not a particularly long run.

There are other pipe options than copper which provide better choices for retro-fitting below floors and above ceilings.
Pushing 3m of copper pipe beneath a floor means that you must have 3m of 'space' from which to push it.

PEX/Al/PEX is more commonly used for underfloor heating (UFH),
but is lightweight and can be pushed through a void easily.

It's semi-flexible, and lengths greater than 3m are supplied as a coil.

PEX means Cross-linked polyethylene, and the Al is for Aluminium between the two PEX layers.

Plumbing and heating outlets like BES offer a range of lengths at different bores, and fittings to convert back to copper pipe or brass-threaded couplings.
32mm pipe with a 3mm wall thickness each side gives you an internal bore of 26mm.

PEX/Al/PEX can be bent by hand, and the aluminium ensures that it stays in that shape.
With ingenuity you can guide it around corners!

Remember to slide on a length of pipe insulation before you put fittings onto it.

When fixing floors and walls in place I always leave lengths of polypropylene string to act as pull-throughs for cable at a later date.

Tie each end around an old nail to prevent it accidentally being pulled into a hole...
... and label the ends with PVC-tape.

Colours help to differentiate strings emerging at the same spot.

The above set is from Amazon.
Other rivers are available.

I think thats what I meant by the suggestion to fit an electric element to a towel rail?

Thanks, we're planning on a large normal radiator. Heat Punk suggests that we need a 1800x600mm K2 radiator for a 45 degree flow temperature, but it's possible I've done it wrong! It's a 2x2.5m room with two outside solid brick walls, and I'm adding 70mm of PIR. I'm assuming that a TRV will prevent the room overheating while we insulate and upgrade the radiators in the rest of the house.

That is what I originally planned, then I fell back to 'ignore and rely on heat from adjacent room and steam when the bath is in use.@

The TRV will indeed prevent any overheating.

I had been considering turning the old 35mm copper pipe into a heated towel rail. If I do, it sounds like I should just use an electric element and not connect it to the heating.

Not sure about this!  Personally I would either recycle the pipe or turn it into a sculpture.

I had been considering turning the old 35mm copper pipe into a heated towel rail. If I do, it sounds like I should just use an electric element and not connect it to the heating.

Regarding feed and return pipe sizing this cheat sheet gives you the numbers, you may wish to take a view.

https://www.heatgeek.com/does-my-pipework-need-upgrading-for-a-heat-pump-with-cheat-sheet/

Just to be clear, our old bath waste pipe was copper, about 35mm OD and 32mm ID, probably from around 1950. I was just querying it because when I looked at plumbing supplies, I couldn't see any listed as 32mm, only 35mm
Copper Tube - 35mm x 3m - 13595
Wednesbury Plain Copper Tube Length 35mmx3m (Table X) X035L-3
I heard somewhere that 'tube' is measured ID, while 'pipe' uses ID and just wanted to check it was the same thing.

You are right about 35mm, my mistake.

I'll try to clarify the situation for the D2 drain. The airing cupboard is on an outside wall, above our porch and next to some patio doors (yes, it's an unusual old house!) so there's nowhere for a heat pump directly below it. Instead we plan the put the pump around the corner, and would rather run pipes under the floor rather than around the outside. I don't think the D2 needs to be follow the same route. Can the D2 just go through the wall in the airing cupboard, and discharge into the gutter of the porch roof, like the old boiler condensate pipe? (the rain water goes into the foul drain, again probably 1950's plumbing)

The D2 can certainly go through the wall assuming that the porch doesn't conflict given the levels (look at the diagram of a discharge to understand) and of course it doesnt have to follow the same route.  I think many will say you cant discharge directly into a gutter, and you certainly cannot discharge directly into a plastic gutter.  I think discharge into a metal hopper (if there is one) is probably acceptable, but again others may disagree.   You are talking steam and boiling water so safety is paramount. 

Some argue that it cant discharge onto a flat roof because the roof material might be damaged.  Given that the discharge rarely if ever occurs, and most flat roof coverings are torched on, I personally struggle with this argument!  There is scope for interpretation and of course different plumbers do have subtly different interpretations.  That would be the advantage of doing the UVC at the same time as the bathroom refurb, when the HP man comes he isn't touching it so doesn't need a say.

The most common ;#'recognised' discharge points are just above ground level with a protective mesh to stop people putting their hands under it, and into a suitable foul drainage system.  The rules on length can get quite restrictive if you need more than a couple of bends, possibly forcing you to 28mm rather than 22mm.  You really do need to research this if you are doing it in advance.  On the other hand, perhaps you dont need to.  Since it can (subject to levels) go out through the wall the HP installer can fit it without involving the bathroom.