Nailing an argument
The one - and I think I should stress, that because it is the only one, it betrays the inherent weakness of the whole case - good defence of First Past teh Post voting has been the so-called 'constituency link' - the idea that each group of electors has a direct link with a named MP representing a defined political area and interest.
Many people rank that quite highly, and it has some merit. At least, I've traditionally given it credit. This weekend, however, I read an intriguing old book:
Now, he brings forward the highly important point - single member constituencies are - at the least - problematic with regards to the equalisation of the value of votes.
Elections and electors : studies in democratic representation / J. F. S. Ross. - London : Eyre & Spottiswoode, 1955.
Here's how. The UK has something like 59.6 million people, it will have 649 MP's after the next election. Thus there should be (rounding) 92,000 people per MP - on average.
Obviously, practical considerations of geography prevent the simple parcelling up of the country into constituencies of 92,000 people. Town sizes, transport links and historic associations come into play. Thus therewill be variety in the size of constituency, and thus variations in the effective weight of each vote (i.e. a constituency with fewer electors will have as many MP's and votes in Parliament as a constituency with more than average voters).
Now, there are limits to how much they can vary. Ross suggests one third of the average because - taking our numbers above if a constituency falls below 61,000 or rises above 123,000 it would be closer to the average to merge or split the seat as appropriate.
With multi-member constituencies, this value - one third - falls as a ratio of the total size, since the constituency need only be adjusted by one representative in any given circumstances.
Thus large mulit-member constituencies tend towards an equalisation of the value of votes more than single seats do.
I suspect I've not made that clear enough, I'll return to it when I have more time.