Political parties and their accounting units are required to submit their accounts to the Electoral Commission each year. Those with a gross annual income or total expenditure of over £250,000 have six months to do so. The Co-operative Party sent their accounts in six days late and might have to pay a fine of £500. Final accounts have yet to be received from the British National Party and Scottish Liberal Democrats.
13 political parties reported income or expenditure of over £250,000 - the big ones, plus Respect, Sinn Fein, UKIP, Co-op Party...Seven accounting units are over the £250k figure of which four are rich Conservative Associations (Kensington, Aylesbury, Surrey Heath, and London & Westminster)
Further reading. Electoral Commission Press Release...The accounts of the parties and some interesting names in that list, like The Southport Party, Space Navies, Norwich Over Water Party (what?). All in PDF format - looks like they have been quickly scanned from paper copies. Co-op Party Accounts here (PDF)