So, there are a few games. Do I think it would help? I don't honestly think it's such a big deal that you need a simulator. I would be more inclined to practice in a controlled few blocks of low traffic and drive around in that area until I feel more comfortable. Just like driving in the United States/Canada, you can easily be overwhelmed by driving variables you're not used to. Thus, I would recommend starting out with few variables in the UK and progress as you feel comfortable. I would also familiarize myself with the common road signs and traffic laws which may be different.
Start here: UK Driving Safety info
This seems more important to me than trying to change your subconscious left-right hand driving habits.
There is Euro Truck Simulator which involves left hand driving:
Then there's this company which has a UK version of their simulator:
And there's SimuRide which makes a less expensive home edition:
I'm not personally reviewing any of these games for quality / helpfulness. Whether they are effectively going to help is going to depend on the person. These are the only UK based driving games or simulators I'm aware of.
GTA 3 also had a London 1969 mod. I don't know if it changed the driving of traffic from right to left though. Or, if you sat on the other side of the vehicle. But, it might be another game to research as GTA is very sand-box and has a lot of other traffic and working lights etc. should you chose to obey them or break the law a lot as the game more or less entices you to.