The build

Underfloor will be electric. We’re having 2Kw of solar panels as well. Mainly so we’d pass sap. You come across them? Don’t know if that’s just a Scottish thing. As all regs are different up here to England & Wales. Anyway it’s lots of box ticking to make your house as energy as efficiency as possible without being a passive house.

I believe the MVHR will take the outflow from cooker hood. It takes hot damp air from kitchen, utility & bathroom. And feeds fresh warm air into the bedroom & living rooms

Yes we have SAP down here but, ours isn't anywhere near as restrictive as yours. In England and Wales, the property is judged against an average dwelling when it comes to running costs. In Scotland it is assumed that the average dwelling already has some form of renewable energy, so you have much harder targets to reach. 

Now then, electric cabled underfloor heating system, they are super cheap to buy and install about a 5th of a wet system but, they are traditionally quite expensive to run. An average 100sqm home run for 4 hours a day would be about £110 a month for electricity with a cabled system but, underfloor heating systems are not generally designed to be switched off and on daily, they become much more effective when running permanently but at a lower temp, that way you remove the drafts like in a conventional house. However, if you run a cabled system like a wet system it generally works out more like £275 a month. That's assuming 1kwh is about 15p.

I'm not sure how you want to run your system but, your vaulted lounge is going to take some warming if the heating goes on and off throughout the day.

Sorry SC, this wasn't meant to be a downer....