Getting hold of the SH standby asset or a C130 is never as easy as people on those fleets seem to believe, and in all these cases time is of the essence or the SAR fleet wouldn't be involved in the first place. The standby Chinook isn't much good for a job in Yorkshire either. Yes, there is more room inside a Chinook, but space normally isn't a limiting factor for a Sea King doing a medical transfer, indeed they are routinely done from the IOW in an AW139!! If a yellow helo isn't the best asset there are civilian air ambulances available as alternatives which are often easier to get hold of than other MOD assets.
As for the stats game, play it whichever way you want. 12 assets for 65M people is one way of looking at it, 5.5M to 1. However, given that 4 of those assets are in Scotland, you could say there are 8 assets to 60M for England and Wales, 7.5M to 1, or, if you take out the asset in Wales, 7 assets for 57M in England, 8M to 1, or, during the quiet hours, 6 assets for 57M, or 9.5M to 1!
As for 64M not needing SAR or rescue at any one time, I would suggest RTCs, vulnerable misper, etc are a nationwide phenomenon and more than 1M people are at potential risk at any one time, as the map of actual incidents throughout the UK during any particular year will show.
Yes, there are hotspots, mountains, coastlines (especially in summer), etc, but the overall spread of incidents is truly national.