Thinking on the spot about how I would quickly design an electoral system, I would go for an MMP system. I would have two ballots, where you vote for a local candidate and party. I would probably enlarge the size of the House of Commons. I realize that doing that would open up cheap political points for opponents of electoral reform. It absolutely would. Adding more politicians is never popular, but I think that's a cheap shot.
I'm going to give one other reason that's often overlooked. The U.K. has an incredibly large House of Commons. They can't even seat everybody. If you've ever gone to watch it, it's quite remarkable. One of the interesting side effects is that some people have argued this helps to reduce party discipline. If you get elected as an MP and you're in a caucus of 300 people, the odds of your ever getting in cabinet are slim, so you need to find something that you're going to do with yourself if you're going to carve out a career as a member of Parliament. By enlarging the House of Commons, you're enlarging the number of backbenchers relative to cabinet. I think that could have an added side effect and benefit of perhaps encouraging a little more independence on the part of members of Parliament. That may be an added benefit, which often doesn't get discussed. I would probably err on the side of quite a number of adjustments to seats. I'd probably err on the side of being as proportional as possible.