AV would require a bill comprised of one sentence 'voters will order candidates 1-2-3 rather than place an X next to their one choice'. And thats it! All the boundaries could stay the same and it could be in place for the next election.
The problem is getting AV past the neaderthal wing of the Labour party - those Labour MPs sitting in safe seats and then getting past the Tory Lords might take a year. The Labour MPs opposed are mostly on the right, mostly Blairite, although it does include some on the left. Names that immediately spring to mind are; Tom Harries, Jim Murphy, Ed Balls, Diane Abbott. Tom Harries has already said he would vote with the Tories in bringing down AV. Any Lib/Lab/rainbow pact could only afford 9 Labour MPs to vote with the Tories or no more than 20 to abstain. There is a possibility that one Tory MP Douglas Carswell would vote for AV, but that is doubtful without first a referendum.
What are the Lib Dems to do, risk that the Labour party can deliver its right-wing MPs and that a fragile rainbow coalition can hold together, or trust the slimy Tories, go into coalition with them and alienate most of its own party and voters? The other thing to consider, is that any referendum will be hard to win with both Tory and Labour opposition. Most of Labour will support it if they are in government but the ferocity of opposition Tories and their media will be intense. Yet if the Libs go for a referendum with the Tories, both Tory and Labour might be hostile to reform, plus a lot of left leaning ex-Lib Dem voters might show their anger at Clegg making Cameron PM and vote down reform. It is an impossible choice. Personally, being on the left, I hope they choose a Lab coalition. The Tories are only interested in gerrymandering FPTP even more and giving money to their rich city friends.