The story so far: a volcano in Iceland with an unpronounceable name (Eyjafjallajoekull, although strictly-speaking that's the name of the glacier the volcano is under) erupted on Wednesday and the ash cloud has been blown south-east and is now hovering over the UK, Scandinavia and a fair part of Western Europe. Through-out yesterday UK airports were being closed, starting in the north, and by afternoon all commercial airports in the country were at a stand-still (apart from some very small helicopter and light aircraft services.)
The ash cloud is about 30,000ft up but starting to come down. There have been reports of a light dusting in parts of Scotland. Commercial flights won't resume until Saturday (tomorrow) at the earliest. Of course, it is theoretically possible for the eruption to last several years, but no-one is really considering that eventuality at the moment.
So let's consider it.
How do you go anywhere with all the airports closed? Well, Paris is a couple of hours from London by train and the Parisian airports are still flying, although not to the UK, Sweden, Norway, Denmark or Iceland itself, but they could get you to Australia or the USA if you wanted.
Once all the out-of-place passengers have made their way home our main problem with a lack of commercial aviation is that most of our fresh fruit and vegetables are flown in; and of course our airmail post travels that way. The lack of domestic flights will be a nuisance, but you can drive from anywhere to anywhere in the UK in about 12 hours.
The long term net effect on the UK economy could well be positive. It would be a sort of compulsory deglobalisation. Local foods would take over - out-of-season produce would simply be unavailable. Britons would spend their holiday money here rather than on the Med or in Florida. Our balance of payments, which has been negative since the 1980s, would flip into the black. Obviously the City would continue to export financial instruments, insurance policies and the like, with the greatest of ease.
The only problem is: it might well all be over by the weekend.