OK, here we go. Lets start with the easy one first. The Lib Dems will never be in power. Unless in a coalition through a hung parliament, where they *might* be able to squeeze AV out of a weakened Labour Government.
The Tory party will never go for PR, not least because of their ideological objection to it, but because it will more often than not deprive them of a working majority. In some ways PR would benefit them, as it would counter the massive boundary bias currently in the system (the Tories won more votes than Labour at the last election, but Labour have a huge majority, and in the 2010 election a 10% vote lead will only give them an 8 seat majority) but it would forever deprive them of the magical 50% vote share needed for majority.
Which pretty much is where Labour is as well - why go for PR when current boundaries give you a huge edge in the elctoral system, and without it you would never be able to hold power alone? It would also destroy Labours heartground safe seats, where much (in some palces almost half) the voters owe their income solely, through the civil service or through benefits, to the state. If votes inthe South actually counted as much as in the North, Labour would be in trouble.