A Written Constitution
A written constitution with an elected second chamber particularly is the best arguable form of government (seriously, try it). The USA, widely regarded as the World’s most advanced democracy and it runs this form of government mandate.
Mark Semple commented
We do actually have a constitution, but it is absolutely all over the place, it contains the 1622 Bill of Rights, a 1919 act and a 1948 (ish) act plus 'some parts of earlier articles' - that is earlier than 1622, unidentifiable and in bits back to the Magna Carta. It;s not easily understood and needs sorting out.
What is more important than this however is the fact that we do not have an elected second chamber and the Queen owns a large amount of personal wealth in the form of land. If it would be possible to eradicate this problem without dismantling the welfare state as a popular political individualist and possibly objectionist (liberal economic) reaction then I would certainly be behind it.
http://38degrees.uservoice.com/forums/78585-campaign-suggestions/suggestions/1602851-an-elected-second-chamber?ref=title and others for a campaign for a second chamber
and http://38degrees.uservoice.com/forums/78585-campaign-suggestions/suggestions/1602857-the-great-property-swindle-uk-land-registry?ref=comments for some really good points on the state of Britain's land registry and the amount of money tied up in a plutocractically dogged system that could save us some off the pain of cuts