You need to meet the requirement of the Building Regulations. Part L1B specifically for conservation of fuel & power. To save you looking, you need to hit U value of 0.28 or lower for your wall construction.
Kingspan website has an insulation calculator which will give you a pretty decent idea of what thickness you’ll need for your wall construction. Assuming brick outer, medium density internal blockwork and partial cavity fill you’ll likely need at least 40mm of insulation to hit a U value of 0.28 (domestic extension target).
Handy links: Kingspan Calculator[/url]
Approved Doc L1B
Jeff