I am a long time hayfever asthma sufferer - ditch the electrostatic filter - waste of money IMO. There are so many sources of dust and these are effective on a few and not all that much. A bigger problem is the reduced humidity - I added a humidifier to the last two systems and that really did help."
As to ducting - as you seem to have decided go with those who tell you what you can do - and when anyone says 'the standards say . . ." - tell 'em Bloss nags you endlessly that "minimum standards are not best practice" . . . and that ducting will fit fine.
And heating ducting in a ceiling is just dumb - so as you say don't do it.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.