chopjock
I cannot answer your points in terms of the rules and regulations that AA's have to comply with in the UK as I'm not a professional pilot and there are far more suitable and qualified people than me to answer those points (if they choose to do so).
However, your point about we could have far more helicopters if they were single engined doesn't stack up (IMHO). All AA's in the UK are charities and with the exception of one county - and two groups of 3 counties - they are responsible for raising the cash to keep an aircraft flying. To give you an idea (and these numbers are from the 90's so well out of date), the difference between an AS350 and an AS355 was about 20% per hour. Taking this to a logical (albeit imperfect conclusion), an AA charity would need to raise 70% more per month to have two single engine helicopters. Given the current climate (and again forgetting rules and regs) this would be very tough.
The final point is that there simply isn't the demand at the moment throughout most of the UK for each county to have two aircraft. If you look at the amount of calls per week that each does then one is suitable. Take into account that they have to land in city centres if required then one twin helicopter per county is the only answer.
Anyway, just my opinion as a volunteer for one of them