Here's the lounge process, but we've had to do 4 external walls so far.
Bought a Zehnder ComfoAir 350 for £500 (deal). We have a radial ducting plan which involves lowering a (tall) corridor ceiling by 20cm to move against the joist direction... fingers crossed.
Yep, continuous background ventilation with 98% heat retained. No need to punch holes in the external walls (100mm PIR).
We'll be well beyond the performance of an average new build at around £1k materials and many days of faffing! Should pay for itself in ~2 years.