If you can drive in the UK, you can deffinately drive in the states. Automatics are much easier to drive than a manual, and so long as you remember the basics such as keeping your foot on the brake when stopped at lights etc, you'll be fine with an Auto.
Driving on the 'other side' is no big deal. Its rare you'll be on a road that isn't a dual carriageway to be honest. At first it feels like you are sat way over to the left hand side of the car, but the lanes are so wide over there, just line your self up with the centre line closest to you and don't worry about the other side of the car. After a couple of days doing that, you'll feel comfortable about positioning in the lanes.
Driving standards are much better out there in my opinion. Everything is just that bit more relaxed. Other drivers always seem to 'let you in' if you want to change lanes, and because everyone has autos, no one seems to be in as much of a rush as they do on British roads.
If you are really that nervous about it, try to fly to Sanford rather than Orlando Intl. The roads by Sanford are a lot quieter, and you'll have the ever-quiet Expressway to use to get down to I-Drive. Coming out of Intl, which is what I had to do, is quite daunting as it is a maze of roads, but I had no trouble and quickly found the Beeline Expressway. Even though you may need to pay some tolls, it will be much easier if you stick to the Expressways coming out of either Airport. We actually got straight off the Expressway and on to Sand Lake Road, as it was a 'straighter drive' to our hotel (on th corner of Sand Lake and I-Drive), and even that road, depsite loads of traffic lights, was very easy to drive along and quite stress-free even for a first timer.