Or to put it another way, everything it owns outside the UK has an implicit valuation in the market of more-or-less nothing - which, some would say, is bonkers.
No market risk and the implicit (or even explicit) state guarantee: Pay a fixed amount now, and when Junior goes to a state college, his tuition is covered.
Depending on how you look at it, the fact that Fannie and Freddie have such a big share of the market and carry an implicit government guarantee is either a big cause for concern or the only way in which the peculiarities of America's mortgage market can operate with relative safety.