Under Labour, two unforgivably corrupt mutations to our electoral system were allowed to fester like an unchecked cancer, threatening the fundamental health and integrity of our democracy. The first was the unequal electoral quotient, which because it allowed Labour MPs to be elected for Northern towns with shrunken populations with two-thirds the votes needed by a Tory candidate in the South East both Blair and Brown were content to leave be. Most mature democracies work with an EQ of +/- 3%; slightly shakier ones with +/- 5%. Under Labour the UK was way off the scale. To the sound of much Labour whining, Cameron is correcting this.
Secondly, Labour not only introduced postal-votes-on-demand but failed to regulate the voter registration system at a time of mass immigration. Any illegal Commonwealth overstayer can still register unchecked, and add a host of fictional housemates to the register without question. Any Bangladeshi (and yes, those from the Indian sub-continent have been caught time and time again at this so this is fair) can include every member of his extended family, including those not even in the UK, on his registration form without question. Michael Pinto-Duschinsky has estimated that there are some 3.5m persons on the Electoral registers who shouldn't be there - and an equal number missing who should be.
Trying to change to an AV system at a time when the most compelling and fundamental components of the democratic process are in such disorder is pure folly. Get the corrupt mutations sorted first, cure the cancer that's actually killing democracy, before you even start thinking about changing the system.