Brown certainly isn't borrowing from the IMF, though he could try to do so (the last time the British government borrowed from the IMF was Jim Callaghan in the 1970s). The government issues bonds (IOUs, effectively) on the money markets and anyone can buy them - though most are bought by banks and major financial institutions. There are also numerous small ways of the government borrowing from individuals - Premium Bonds, National Savings Accounts, even War Bonds all form part of the National Debt.