This is a link to a recent post on the London forum.
It is written by somebody from the USA, based on recent experiences of driving a rental car in the UK. There are plenty of other suggestions and useful comments in the many replies, as well.
While most of the info is very much specific to driving in the UK, a lot of the advice really holds good for folk who live anywhere where you drive on the right, renting a car and driving on the left for the first time. The specific rules of the road, speed limits, etc and some quirks are very much UK-centric, of course. However, I'd say many good tips apply equally for Europeans or those from north America who drive in Australia, New Zealand and South Africa for the first time.
I hope you find it helpful. For those with other tips to share, add them perhaps to the other post as it will always be linked here as well.