"First past the post" electoral systems have their faults and I wouldn't say they are perfect, but the merits of AV have been over-hyped. The argument that the winner of AV has more than 50% of the support of his constituents is simply wrong. For a start, around 50% of them will not vote and of those who actually do count towards his score a fair proportion may well be second preference votes. Not much of a show of support when Mr Apathy gets 50% first choice.
But, in fact, the results under AV can be quite ridiculous and I give the following by way of an example. The colours used here are for example only and should not be taken to express any particular prejudice. The numbers and the backstory are not particularly extreme and may fit many constituencies.
Imagine a hypothetical constituency where 49.9% of the electorate vote for the Red candidate, 25% vote for the Yellow candidate and the remaining 25.1% of the vote is taken by a motley assortment of Blue, Purple and Violet parties. (OK, 49.9% is pretty borderline but in reality anything from 45 to 50% is not uncommon, and the story wouldn't be very different).
It turns out that the Yellow candidate used to be the Red Party MP, but he defected a few years back when it looked like he was going to be deselected, so as a result no Red party voters will put the traitor down as their second choice, and since the rest of the field are far too Blue/un-Red, most of them don't put down any second choices.
On the other hand, because the current Red candidate is seen as quite a good MP and the candidates of the various-shades-of-blue parties all have reputations as shysters and chancers, nearly all of the voters who put the Yellow candidate as their first choice put the Red candidate as second choice. Perhaps they feel guilty about the defection.
The Blue, Purple and Violet candidates realise that they each have no chance of winning and tell their supporters to vote tactically and put the other blue-ish candidates as their second and third choices, the Yellow party candidate as their fourth choice but to omit the Red candidate. Voters being like sheep and not wanting to be herded don't do exactly this and a few (say 0.2%) actually put Yellow as their second or third candidate.
So the Red candidate is the first choice of 49.9% of voters and the second choice of 25% of voters (i.e. first or second choice of 74.9% of the voters), while the Yellow candidate is the first choice of 25% of voters and the fourth choice of 25.1% (i.e. first to fourth choice of 50.1% of voters).
Looking at it in terms of which candidates voters don't want to win, the Red candidate has been omitted from the voting slips of 25.1% of the voters, while 49.9% of them have not voted for the Yellow candidate
The Red Candidate has twice as many first preference votes (49.9%) as the Yellow candidate (25%).
The Yellow candidate has been rejected completely by almost twice as many voters (49.9%) as the Red candidate (25.1%)
Who wins under AV? The Yellow candidate, of course, because AV is so much more "fair".
FPTP may have its faults, but nothing as egregious as this.