Well, I can definitely say it was easier for me. The rationalle being that there's already all the silly little things to get used to - like how you steer, looking in the mirrors every time before you signal, using the emergency brake, etc. etc. - that piling all of those on top of the nerves involved in driving for the first time here might add to the stress more than it has to.
It didn't take long to figure out that normal everyday driving here is really not very different than driving in the US, and being able to figure that out without an instructor sitting next to you telling you everything you're doing 'wrong' seemed to really help.
For me, the fact that I was already comfortable driving meant that it was much easier to add on all the other little things I had to do to pass the test, and therefore saved a lot of money and time in lessons.
With the exception of it being "not much different" driving here (I think it's
very different in some ways) I agree with this -- getting in lots of "pilot hours" behind the wheel all by yourself actually confers a confidence and loss of any initial fear that is invaluable to focusing on the rest of the "little stuff" that is new and petty and has to be learnt for the test.
Having said this, I failed, but on one major f*ck-up on a very nasty and complicated roundabout -- even my examiner said the rest of my driving was near-perfect, strong and confident, which I know came from driving every single day by myself until I got over what had been an initial bout of numbing fear.
I was forced to drive by myself because of the job I held at the time, and at first I was white knuckling every single day and hating the entire thing. But over time I went from "I hate this, I don't want to ever drive in London or the UK again once I quit this job!!!!!" to "Oh my God I'm having a blast driving again!"
After getting over my fears (and London traffic can be pretty high in the fear factor) you couldn't keep me out of the car alone. I know now that it will stand me in good stead when I take the test again, because at least I'm not a nervous driver here but one who came to enjoy it here as much as I used to in the US.
If you get back your comfort level by throwing yourself out there alone a lot, it is so much easier to "add on" all the other things you have to learn for the test, as Robert points out.
If I hadn't gotten rid of that fear by just getting out there, I would have failed on MUCH more. Which isn't much comfort. But I still believe driving by yourself if you get the chance does amazing things to help. You do have to be self-disciplined about keeping up the new methods rather than driving the way you would back in the US. But it's still of great value, in my opinion.
Plus, surprisingly it's actually EASIER to focus calmly on everything you're doing, observing, and where you're going, WITHOUT anyone in the car yammering your left ear off.