At the moment it's about £1 per litre wholesale so if you put a 20p mark up and then VAT you end up with about £1.40. This is for smaller airfields - some airfields though don't put a mark up for their members so sell at pretty much wholesale + VAT eg Fenland where it is £1.17 per litre - possibly where Stik goes?
Obviously at larger airports where Shell etc sell their own fuel directly then they don't (or shouldn't) need to put the mark up on so you end up with about £1.18.