I'd vote for a 60mph limit on the motorway while we're at it.
Well I'd vote for a 20mph limit in towns and an 80 (or maybe 85) limit on the motorways, given my totally unresearched opinion is that the 70 limit feels too slow for a lot of drivers, hence decreases the respect for speed limits as a whole and makes people more likely to speed in more important speed limits. But we've done motorway limits and I'm bored with that, so let's move on.
Despite the fact it would inconvenience me a bit, I'd vote for a blanket switch from 30 to 20 if that was the only thing on the table. Checking Google maps it would add ~1.5 minutes each way to my commute given how far I tend to do at a steady 30mph (and that the waiting time at the junctions is independent of how fast I drive either side). As much as 4 minutes on a journey I make occasionally on the same basis. Would happily sacrifice that for the real improvement in road safety. Though as has been suggested, better would be for some major urban roads to remain as 30 limits (strange to have such a consensus on a driving issue). That would probably actually change my commute route, as I'd imagine some of those roads would be 20, whilst the alternative would be 30 (come to think if it, given the small difference in time maybe I should go that way anyway as I'd be on roads with less ped traffic).
Can't agree with TJ's 20/40 suggestion - plenty of roads I can think of where 30 would be OK (in a world where most urban is 20), but not 40. With him on the empty streets thing though.
One important point I don't think I've seen is that 20 limits encourage people out of cars onto bikes - both because there's less perceived speed advantage to the car, and also because the lower speed differential makes cycling seem safer, and the roads a generally safer place.