What I do is just give up to 2 domestic rates on my listings and have a standard note near the bottom of the listing asking non-uk buyers to ask for a p&p quote, otherwise it gets far too complicated unless you are selling 100's of the same item.
Some bidders ask some don't they look at your profile and accept what you invoice them for. Others will play dumb and expect the domestic rate to apply for airmail half way around the globe. Just make the note clear and stick to your guns.
