Just take the modelled contribution from the MVHR away from the modelled contribution of the main heating system.
If ASHP, then up the capacity to the next available size that is 30% of so oversized.
Probably best. Though there may be that things where a builder can do it under notice.
As it is only a metre high, it may be exempt.
https://www.pavingexpert.com/featur03
It is the same as calculating heating loss from ventilation.
Mass of air [kg] per time period [h] time specific heat capacity of air [kJ.kg-1.K-1] times temperature difference [K]
Walk away from them now, it is going to cost you.
DHW and Space Heating are different things, at different times and at different temperatures. They should be separated.