Good advice if you can do it, but this isn't possible for most people, for example if you're renting or you're in an apartment.

It is called recuperator. If it is possible to retrofit i suggest to use mounted in attic (it will be silent). Otherwise, if you want higher quality not and these `breathing` types the box will have similar size like air conditioner (search: mitsubishi vl-100eu5-e)

Indeed, but a traditional MVHR would require ductwork which isn't common in the UK and often there isn't really any space to route it.

I have been investigating easier to retrofit decentralised MVHR systems such as the Prana Recuperator, Ventaxia Tempra and the Blauberg Vento. They can be noisy (not a problem for me, I don't mind noise) and quite expensive as you have to fit a number of units in different rooms, but still easier than retrofitting ducting.

It's not something you just add. It's a capital renovation. Requires ducting. Tons of homes don't even have ducting.