AOPA US probably doesn't let you put in a UK number because AOPA UK is a separate division to handle UK members, however I don't know what services AOPA UK offers, and I don't know if they offer rental insurance for flying in the UK.
Welcome to AOPA UK
Look at it this way, some flying schools in the US offer heir own insurance of you pay extra for each flying hour. If they offer you $10 per hour or more for the insurance, it comes out more expensive than getting AOPA's yearly coverage for renting in the US.
AOPA Insurance Agency +1 (800) 622-AOPA (2672)
AOPA Aircraft Insurance