You pointed out the inherent corruption on your own. An MP in the proportional system isn't really accountable because he owes his position to the party that put him on the list, not to the voters. The average voter is not likely to know anyone beyond the top 10-20 names in the list, with everyone else on it being party fodder. So, the voters who supposedly picked him/her likely have no idea who they actually picked. On the other hand he/she has no idea who his voters are, since everything about his political career developed through internal party politics - which is how he got on the list in the first place.
@Walsingham: I dunno, Earthworm Jim says all lawers go to hell.
And, unlike a FPTP system his or her mandate from the people is guaranteed to tangibly exist.
I also don't see how FPTP is free from the same problem. Brown anyone ?. Would he have gone on to be PM without favouritism and party politics. Isn't that the whole reason he got where he is. Promises that he was 'next in line'.
A federation type electoral organisation is less democratic because it is entirely possible for major sections of the electorate to be over, under, or un-represented. Maybe you need that in countries that are so big that local identity rivals national identity (the US for instance). Is that the case with the UK. I don't think so.