Not that many places have edge coverage. I think it's because they only really invested in some form of edge network to get the original iphone exclusivity and didn't cover the nation with it as they'll have known that in a short period of time something operating on 3G would come along and made edge redundant, which it basically already was when they started to create their edge network.
I think that o2's current aim is to get HSDPA up and running in as many places as possible because that's where the revenue is (it's what gives decent speeds for mobile broadband).
It can be pretty hit or miss with networks and signals depending on where you stay unfortunately. If I go visit my other half I won't get any signal at all on o2 or Vodafone but will with t-mobile and orange. Where I used to live up north t-mobile and orange were pretty much useless in the whole area but o2 and Vodafone were fine.
I really think o2 and the other networks should follow vodafone and bring out femtocells. I'm sure it'd make customers so much happier with their products if they knew that they could come home and benefit from a 100% signal allowing them to use their handsets to full capability.