Overstretched and they know it
In Southampton at least O2 aren't taking new customers on several exchanges - their availability checker says you can only get "O2 access" which is a pricey re-sell of BT's service on both the Shirley and Southampton exchanges... which is odd as I've had "propper" O2 for the last year from the Shirley exchange!
Wish they'd just fess up to being overloaded....