I’m trying to pay for the VPN through the app but it is only offering it to me in Euros and I want to pay in UK pounds. Is there a way of doing it? I am in Spain for a while and don’t want to wait until I get home.
Is the conversion rate that bad…!?? ;D 
Contact support: https://support.avast.com/support/tickets/new
Try it through the avast website or find a reseller in the UK that is offering it.