Via Megan McArdle (from a good post on why it's hard to leave the euro), we are offered this list:
Bank assets as a percentage of GDP
United Kingdom 389
(For comparison, total banking assets in the U.S. are equal to approximately 82 percent of GDP.)
File under "Too Big To Save." Do be a little careful, however, since countries such as Ireland have financial institutions based there, for tax reasons, without the Irish government feeling responsible for them.