Have you any idea how mountainous South Wales is? Yes, there are valleys, but they are quite steep. You should look at the gradient profiles of some of the railways round there. GWR men used to joke about the geographical coincidence of one particular line ending by the Abergavenny Lunatic Asylum.
What scares me most about the population spread into the British countryside is not the density. Or rather it is. If everyone wants their acre of the 'Good Life', then it's a pretty quick way of ending up in densities that make providing things public transport economically unviable. Much like the villages in the Home Counties that are only half miles apart, but have little transport beyond private cars between them. (Incidentally, the villages that I grew up in in Humberside were similar but seperated by several miles.) Also, in these villages, services such as shops, post offices and pubs are closing all the time, because they can't get enough money to stay open. From an economic point of view, this is a good thing. From a social and environmental point of view it is a minor disaster. The economic argument is not necessarily the only argument, it is just people tend to assume it is because it gives the immediate, short term benefits.
Finally, I agree, power from Whitehall is a good thing - we just need alternative power bases set up before the crash.
At this point, I would like to point out that I am not stockpiling food or weaponry, or for that matter building a shelter in a remote place, to escape/prepare myself for the bad times. I'm not sure I could say that if I had the money and the contacts, tho'... :-$