Bodge alert! My mate who is a plumber put my shower tray on a bed of decorators caulk (3 tubes if I remember). No movement in the 12 years I had the house. Lay it down on 2 thin battens then pull them out.
As others have said, the height is measured from the highest natural ground level adjacent to the proposal(s).
If you really want to go down the route of making sure the height is acceptable in PD terms, then look to get in touch with your LPA. A Planning or Enforcement Officer would carry out a site visit to check the necessary regulations.
Pure X recirculating is £2720 + VAT, and Pure is £1890 + VAT.
All Bora range is going up in price as of Jan, and the above will be £2990 + VAT, and £2180 + VAT. Date of your order will secure a price, i.e. although not deliverable until March, you can benefit from 2020 prices if paid before end of December 2020
Oh, and they come with filters, etc so you would not be venting fats back into the MVHR system. Naturally, if you only want to use MVHR to vent the cooking, then perhaps that is not a good idea, and would (almost certainly) not pass Building Regs.