I had considered removing and replacing the flooring but when I looked at it it looks like the partition walls and all walls were put in after the flooring so being able to remove the large sheets of chipboard is going to be tricky which is why the overboarding option appeals.
With chip, I would advise taking it up & replacing; it’s surprisingly easy once you get one bit up & you don’t have to remove it in sheets, just cut around the edge or the room (check for pipes & cable) & break it up into manageable chunks with a lump hammer & bolster chisel!
Not sure on the tile size as I don't have one with me to measure but think they're about 500mm x 300mm.
Large format tiles need a very flat tile surface & you will need a large format trowel. Polished porcelain also needs sealing but some come pre-sealed, you need to check with the supplier. Flexible cement powder adhesive only on floors but depending on your tiles & tile colour, you may need white adhesive to avoid bleed through.
When it comes to putting the aqua panel in against the stud wall, will this need to be skimmed
NOOO don’t do that; plaster is not ideal when tiling in any case as it reduces permissible tile weight but Aquapanel can’t be plastered in the conventional way (well not quiet).
can it be tiled straight onto after applying the acrylic primer?
Yes but Knauf (Aquapanel) recommend not to prime & it’s not necessary unless your adhesive manufacturer recommends it. The adhesive will depend on the tile size, don’t use tub ready mix on anything much bigger than 250mm.
Do the joints between panels need anything doing to them?
Fit the panels to correct way around; Knauf have their own joint sealer but some use silicone; you need to be careful as not much sticks to silicone including tile adhesive, if you spread it around too much it’ll cause adhesion problems. Just before tiling, I fill the gap with adhesive & apply reinforcing tape over the join working more adhesive well into the tape mesh; as far as is possible try to ensure the gaps in the tiles don’t coincide with the gaps in the Aquapanel.
By all means continue to ask questions but your starting to make some wrong assumptions which will lead to you making mistakes. Again, I would urge you to read the tiling sticky & forum archive posts.