Which of these parties would you vote for?

[Erc]

He said 70% of seats. Not 70% of votes.

Even more conducive to fraud by the majority party. I would point to a few real world examples of majority parties winning 70+% of the seats in their parliament:

http://www.electionworld.org/singapore.htm
http://www.electionworld.org/azerbaijan.htm
http://www.electionworld.org/belarus.htm- President Elected with 70+% of the vote
http://www.electionworld.org/cameroon.htm
http://www.electionworld.org/chad.htm
http://www.electionworld.org/egypt.htm

That is just a small sampling. Most of them do not have Freedom House ratings, in either poltical rights or civil rights, less than four.

True enough but if (say) the North East region of England were to break away, based on who holds the Westminster seats at the moment, something like 90% would be held by one party.

Well, parties do tend to (in the long run) adjust themselves to the countries they're in (on the national level).  If you imposed the US Party system on the UK, the Democrats would always win...to a point where either the Republicans would move left or the Democratic party would fracture.

You can even see this on the local level, on occasion...Phil Bredesen (D-TN) is probably more conservative than Arnold Schwarzenegger (R-CA).

So, if the Northeast were to split off, Labour would have the immediate advantage...but I doubt Labour could actually keep itself intact for that long without external threats.