OK. Let us manfully resist exploring the comedy value inherent in Simon "the straight choice" Hughes accusing anyone else of lies and nastiness. And let us set aside the chutzpah of the Lib Dems – the party most notorious in British politics for its deceit, distortions, evasions, half-truths, and dishonourable tactics – proposing to report other political organisations for made-up facts and underhand campaigning (presumably calling your opponents a "right-wing clique" when they include more than half of Labour MPs doesn't count?). Let's just focus on two things: are the Yes campaign's claims straightforward facts; and are the No campaign's claims lies?
Two high-visibility claims made by the Yes campaign are that the introduction of AV will end safe seats, and that it will force MPs to be more assiduous in their expense claims. How will AV end safe seats? Presumably few of the 216 seats safe at the May 2010 Election because the candidate receives more than 50% of the vote will cease to be safe. What will happen instead is that a new ("type 2") form of safe seat will emerge – one where a candidate from a party that, under FPTP, would come second (or even third), positions herself politically such that she always secures the vast majority of second preferences of the candidates coming below her, making the seat safe. In advance, we have no way of telling whether the number of seats made into new "type 2" safe seats would be greater or less than the number of currently safe seats under FPTP that would cease to be safe under AV – it would all depend on how the politics played out.
The claim that there would be fewer expenses scandals under AV is so laughable as to be barely worth countering, but it appears to rest on the (false) idea that expenses mis-deeds were more common amongst those in safer seats. Alleged wrongdoing – and of course I'm sceptical that many of the MPs actually did anything wrong at all, but that's another story – was more common amongst those that had been in the Commons for longer, not those in safe seats except insofar as those in the Commons for longer tended to be in safer seats, for obvious reasons. But since AV would merely create different safe seats, not eliminate safe seats, the whole argument is a red herring.
So the Yes campaign is relying on some made-up facts of its own (or, to be overly generous to it and express matters in civil service-speak, on some "highly contentious propositions"). Of course, that wouldn't excuse the No campaign if it were itself engaged in mendacity. But is it? Most of the fuss seems to revolve around the claim that AV would increase the potential voting influence of BNP voters. Well, that's just a fact, an objective indisputable property of the mechanism. AV increases the potential voting influence of all those voting for minor parties that do not secure seats under first past the post. That's in some sense the point of the mechanism! At the 2010 General Election there were 35 constituencies in which the majority for the winning candidate was less than the BNP vote – meaning that (if FPTP votes reflect first preferences) the candidate coming second could, in principle, have won by securing second preferences from BNP voters, if she could have done so without losing her first preference voters. That's not a made-up fact or blanket untruth. That's just the way it is. (Of course, matters are more complicated in that there would also be second preferences from other kinds of voters, but however you cut it, the reality is that in a non-trivial number of seats victory would depend on securing BNP second preferences. That is as close to a fact as anything gets in psephology.)
Thus, the Yes campaign has zero basis for complaint – indeed, there is rather more basis for the No campaign complaining about the untruths of the Yes campaign. And of course the No campaign does do that – but in the proper and conventional way, by trying to win the argument with the voters, not by shrill, unfounded, and desperate complaints of unfairness to electoral authorities.