Deep Retrofit

Hi folks

Looking for some advice. Proceeding with a deep reno in a house which requires a lot of work and trying to keep to a budget. We're needing to make compromises as expected due to the price of some quotes we are getting back. Originally thought the way to go was external insulation, heat pump and DCV but the prices for external insulation in particular were extremely high. We're now set on cavity fill (with the intent to do EWI later down the road), attic insulation, triple glaze glass/doors, block up open fireplace with log burning inset fireplace, new condensing gas boiler w/rads and as much air infiltration mitigation measures as possible.

The last two things we're looking at is the existing suspended timber floor and ventilation. Haven't got quotes in yet but we're already looking tight. Ideally would want to remove the floor joist and install a concrete floor w/ UFH and assuming then the house is relatively air tight (as I assume the existing floor is a huge contributor to external air infiltration), go with a DCV system. Alternatively though, we could install insulation between the joists and use an air tight membrane overtop or just install an air tight membrane on top of the subfloor w/ no insulation. We could also move from DCV to passive ventilation.

Don't have an architect involved so just looking for online advice but where do you think I'll get the best bang for the buck etc.?