You have arrived in the UK and are handed a car key and a rental car. I will now describe the four phases of driving on the wrong left side of the road so you can be prepared when this happens to you. Keep in mind that the will be a stickshift so if you need automatic, make sure you let them know ahead of time.
Most importantly, don't panic. Just take it slow and take your time. UK drivers are really really friendly and generally pretty patient.
Phase 1 -- Utter panic. Say the phrase STAY LEFT! out loud as often as you need to. This lasts about two days.
Phase 2 -- More at ease. Your right hand will be bruised as you bash it into the door looking for the gearshift. The mirror positions will feel all wrong and it will be hard to know where to look for surrounding traffic. You'll laugh everytime you reach with your right hand for the seatbelt that doesn't exist over your left shoulder. You will walk to the wrong side of the car to get in. This phase lasts 3-5 days.
Phase 3 -- Ease and comfort. You've been driving for a few weeks now, every day. It seems perfectly normal. However, this is the most dangerous time. Because you feel at ease, you are not thinking about driving anymore and your old instincts kick in. At an empty roundabout you will feel the urge to go right instead of left. You may forget to look to the left for cross traffic, etc. During this phase it doesn't hurt to pull out a few "Stay left!"s from time to time. This step can last for weeks.
Phase 4 -- Native driver. Effortless, nothing bothers you and you remember to fold your side mirrors in when you park. This takes months/years.
I am currently in Phase 3 and don't have much more time to get to the native level. I have enjoyed the challenge. Turned out to be much easier than expected. The only thing I can really complain about is the street signage and street names. Hard to see street signs, if they exist at all. And streets tend to change names A LOT. Within one mile a street name can change two or three times. Not saying it's bad, just different.
While I share a car with my colleague, he's never driven and that is fine with the control freak inside of me. I am a spectacular driver and navigator and got us to set in the pitch dark every morning, only having to check directions once or twice. We drove a crew member home the other night from set and she was really impressed that I knew where I was going when she told me where her car was parked (near the production office). I was impressed with myself too.