How do I bathroom / kitchen / extension? etc.

I basically am stripping the entire thing back to brick, but I definitely don't have a £500k budget!

You really don't need to spend that much or strip back to brick for EnerPHit though. Remember we're planning on external insulation, so in theory you just wrap your house in EWI, use an external airtightness product like the purple gunk I linked to and suitable windows and doors and job's a good un. In theory.

I saw a good case study today which was social housing where they used Passive Purple on the exterior, covered it in insulation and the tenants didn't even have to move out.

I think our practical problem will be the loft roof - will be difficult to get enough insulation in without going higher than allowed. And fundamentally I'm not sure how much I care for certification, especially if I have to do things like spend several hundred quid on Excel files to achieve it!