KARFA

I am a bit confused why it would cost significantly more in £ than the local currency for an exEU?

Basically as you noted ba,com prices using the local currency of the departure point. If you call BA they often default to pricing it in £ for UK members but it is just a matter of being converted using the standard exchange rates from the local currency. On that basis the price offered in £ should just be the converted local currency amount.