So the current electoral system for UK general elections is First Past the Post (FPTP), more specifically it is used in the UK's 646 constituencies to elect MP's and the party with the most MP's elected is asked by the Queen to form a government. FPTP works on the basis of, whichever candidate in the constituency gains most votes overall, wins. Simple. That being the major benefit of this system, it is simple and easy to understand, it is the status quo and the public are used to it. However, since the candidate only requires one more vote than the runner up, he/she may not have 50% of the vote, and therefore cannot be considered a legitimate winner. And since when it comes to forming the government from parliament, FPTP dictates that it is the number of seats possessed by each party that counts, not the vote. The have been two incidences (1951 & 1974) in UK election history where the party with the most votes did not become government due to the FPTP electoral system. This is perhaps the biggest disadvantage of first past the post system is the simple fact that votes do not translate to seats.
The is where the alternative vote comes into it. The proportional representational system more accurately reflects the votes cast by allowing voters the opportunity to rank candidates in order of preference. After the first count the candidate with the least amount of first preference votes is eliminated and their second preferences allocated. This continues on until one candidate has over 50% of the vote. Overall that means the government would have over 50% of the country's vote, giving it legitimacy. AV does not however necessarily help smaller parties or independents, since they are usually the candidates with lower first preference votes, and thus are eliminated. As AV is proportionally representative of the public vote, it is unlikely to give a clear majority government, as the public don't usually give 50% of the vote to a single party.
So that was basically a detailed, yet condensed summation of some of the advantages and disadvantage of each system. A lot to think about for thursday for sure!