1. Overcrowding - Westminster is too small to be a practical UK parliament. If all MPs attend the House of Commons (a very rare occurrence), many of them have to stand and the Lords isn't much better. If Westminster only has to house English MPs, there would be plenty of room.
2. No jurisdictional uncertainty - we wouldn't have a situation where non-English MPs have to be asked not to come into the chamber or being turned away from the lobbies because they won't physically be in the English Parliament.
3. Less London-centric power structure - Leeds is geographically in the centre of the country, but perhaps more importantly is in the centre of a swathe of northern cities that are essential to the long term socioeconomic wellbeing of the UK. Moving the nation's governmental centre there would go some way to rebalancing the current imbalance in England and the UK generally. Admittedly some improvements to infrastructure would also be required, such as a high speed rail link from Liverpool to Newcastle via Manchester and Leeds, but this is required anyway.
4. Modernisation - Westminster is steeped in history and constrained by it. A modern democracy needs an open and accessible parliament. The modern communications infrastructure required for this could not easily be retrofitted into a building so utterly wracked with listings as the Palace of Westminster. It would be easier and cheaper to build a new parliament building, fit for purpose in the 21st century.
Anyway, that's my proposal. People will say it lacks detail - what powers and responsibilities would the UK parliament have for example - but unlike David Cameron I don't think it's my place to impose an entirely revised constitution on the people of the UK. I simply wanted to posit the idea of an alternative to the bickered fudge that the current establishment will end up dumping on us for Christmas.
