How hard is it to change?

I am from the United States and I am really used to driving on the right hand side of the street in a left hand drive car. How difficult is it to get used to driving in the UK where it is usually right hand drive and the left hand side of the road that people drive on? Would it be possible for me to rent and drive a left hand drive car while I am there, or would this not be allowed? I really have to look into the regulations so I can see what my options are for my trip.

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