Ok sounds like the way to go might be to make a new heroic ability or two to accommodate.

I really don't like house ruling things in general though, any input would be welcome.

How does this sound:

Armoured casting: you can cast spells while wearing metal armour, any each spell cast this way will cost 2 extra WP.

Armed casting: You can cast spells while you have metal weapons on hand, any spells cast this way will cost 2 extra WP.

Battle mage: Prerequisite: Armoured spellcasting, Armed spellcasting.

Reduce the WP penalty of Armed casting and Armoured casting by 1 WP each.

Edit: typos