Dry lining and pumping cavity walls

[BTwisty]

The following answer to a very vague question is, by necessity, general in nature.
In our relatively mild climate I would likely pump the empty cavity (assuming it is not a timber framed house), find and address the low hanging fruit with regards to air leakage (actually do an airtightness test first to find the issues), seriously consider a simple whole house ventilation system but would likely not add iwi to the mix for various reasons.
Contrary to general perception, quite often the actual windows & doors are NOT the main cause of air leakage in houses and this surprises people when demonstrated why (Anecdotally, window replacement is recommended in less than 1 in 10 existing houses I test/survey for air tightness)

Thanks, yes sounds sensible to leave out the internal insulation incase it causes more problems than it solves and focus on air tightness. Not to mention the expense. We are indeed getting a MHRV installed as well. Hopefully that will get us heat pump ready.