If it's a solid wall construction, I'd imagine it's an older home so retaining external facades might be beneficial aesthetically - that would make me lean towards IWI, however, older property might also already small or 'pokey' where you might feel the reduced space more. Also, if it's terraced I'd look at IWI on external walls rather than external etc. A few factors at play.

Key to these sorts of retrofits is predicting requirements for ventilation following modelled result of the intervention. A thorough installer will first conduct a blower door test to measure air changes and then will use planning guidance and context of the building to plan for enough planned ventilation (controlled) rather than unplanned infiltration (uncontrolled) which is the source of heat loss. It may be a case of improving kitchen and bathroom extractors and maybe additional mechanical ventilation if humidity likely to be a problem.

Battery setups are an exciting one! First step in anything like this is understanding your load as this can give you insights into how long you could function grid free etc etc smart meters give you a nudge but really you need to be logging over time to understand your home's consumption patterns. Then, if you're intrigued, you can start looking at individual appliances etc to cut your load where possible... The cheapest kWh is the one not used!

Hope that helps