Thanks very much — I’m glad you enjoyed the piece.
I’m not a fundamentalist on AMS vs STV. I’d happily have either, and I tend to think STV would be the best system for local government. My most pressing reasons for backing AMS for the Commons are pragmatic. To get people to back change, you need a clear rationale and a hefty dose of reassurance. With AMS, most MPs would be elected as they are now and there’s only one change — the top-up MPs, who I’d have representing relatively local areas. There are voters who like the idea of knowing ‘the’ representative for their particular area, and AMS can accommodate that. It also avoids the issue of preferential voting. I’ve no problem with it myself, but I vividly remember trying to sell it on phones and doorsteps in 2011. People didn’t get the concept, and if they did they often didn’t like it.
I think there are certain parts of the UK — you rightly cite Cornwall, and I’d add Northern Ireland — where STV might well work more logically for the House of Commons. But in many other areas, constituencies of 3–5 MPs wouldn’t necessarily be all that logical either. Surrey is a good example — a localist AMS could have top-up MPs (county MPs, if you will) covering the whole area, which would actually make more geographical sense, whereas you can’t really have an 11-member STV constituency. The sheer size of multi-member constituencies in parts of Scotland, with no single-member element, would pose major challenges. Overall, I think AMS allows a reasonable set of fairly local top-up areas, combined with traditional single-member seats.
Basically, I think it’s best to go with the grain of the UK’s (well, Great Britain’s — Northern Ireland is different) political culture in designing a proportional system. I think AMS is less of a shift and more in keeping with what people are used to, and so it’s more likely to be accepted.