Okay, first a confession: I cannot drive a manual transmission vehicle; never learned and didn't need to know how...until now. And after fifty years of driving it's rather late in the game. But per Rick Steves' suggestion I am renting a car in Durham UK to drive to and along Hadrian's Wall. I have contacted several rental agencies who are happy to take my reservation but cannot guarantee me a vehicle with an automatic transmission. Without one I am grounded in Durham. What is the likelihood of my being able to get a small/economy rental vehicle with an automatic? My cuirrent reservation is with Enterprise.
The operative word here is "guarantee". We heard so many stories about booking an automatic transmission and having to accept a stick shift when they arrive at the car rental place. I wouldn't trust any of them.
There's no guarantee, no matter what the promise. It seems like just about every trip I see somebody in that fix. If there's one available, you'll get it, otherwise, tough stuff. At a major location you can walk over to the next counter and generally get satisfaction at the same price. Durham ain't a major location. Fifty-fifty at absolute best.
I would simply try another company. I have been able to reserve automatics in the past; it was at an extra cost. RS books will list various auto rental companies in his book; try another until you find on that will reserve it for you. I can drive a manual but if I needed an automatic, I wouldn't rely on a maybe.
I would agree with Ken, shop around until a company will guarantee an automatic for you. Absolutely no issue with you not being able to manage manual, plenty of others the same. Another consideration is that you will be driving on the other side of the road, again a good reason to stick with what you know. Ironically we have had the opposite experience ...booking a manual and getting an automatic which proved to be a blessing as it gave us one less thing to think about when driving on unfamiliar roads etc.. We have usually used autoeurope.
I'd rent at a larger airport and drive from there. Moreover, in Europe automatic transmission is somehow a premium feature of cars that is more common in higher category vehicles (not necessarily physically larger). It is unlikely you'll find automatic cars on Economy or Compact categories.
Contrary to Andre's post, I have gotten automatics in the Economy/Compact category every time (4) that I have gone to Ireland. No idea if it is the same on the main continent.
We reserved an automatic from Hertz in Florence because we were planning on doing a lot of hill towns etc. (a manual is just a pain in some circumstances like oh, San Francisco). Well our automatic was ready but the two other couples traveling together could only be offered a manual (despite their reservation). Not one of the four could drive a manual (and yes we switched - we farm girls know all about the nuances of driving a clutch). So forget the small economy rental and pick up your car at the airport where the stats are in your favor. You'll have enough on your plate driving on the other side of the road. I always post a note on the dash when driving in the U.K. - RIGHT SHOULDER TO THE CENTER LINE. Practice a bit around the airport. Oh, and because the roads can be very narrow the street may be very parked up on one side forcing you to the middle of the road but be not afraid. Everyone else is clued in and will be hugging the curb in the oncoming lane.
Like Nancy said, we never had problem getting smallest car with automatic but we always rented near big cities, Paris, London, Edinburgh, etc. Automatic is the easiest way to travel when on the "wrong" side of road anyway it is for me. Had manual twice and went from first to fourth gear each time without even trying.