Sorry: I used "inside" and "outside" to try and avoid confusion! In UK we also tend to refer to the slow lane as the inside lane, even though technically it's on the outside of the highway.
On UK motorways, unless you are in solid nose-to-tail traffic it is illegal to pass on the left/kerb/passenger side. It is only permitted to pass on the right/median/driver side. (UK drivers sit on the right of the car). So a UK driver on a relatively uncrowded motorway does not expect someone to come past rapidly on the passenger side.
I also found Charmion's experience interesting: "It also took a while to stop reaching for the gearshift with my right hand -- and opening the driver's door instead." I had a similar experience the first time I drove a LHD car. My left hand, looking for the gear lever, found the handbrake (oddly placed between driver's door and driver's seat on the NSU Prinz) and pulled it on much to the following driver's (and my) surprise