It was a bit intimidating, even for the locals.
I never did too bad with the left hand side thing, even in cities. It was when I was out on a two lane road in the country and had to move over the right side for one reason or another (passing parked car, etc.) was hard because I sometimes found myself staying on that side out of habit....hard to break.
I worked in Bristol for about 6 months in 2003. The initial drive from Heathrow on a Sunday (after the business-class redeye)on the M4 definitely got my attention.