On the M25 at junction 5 don't keep with the M25 signs but take the M26 eastbound straight ahead towards the M20. This is actually the main flow from the traffic lanes on the fast side, M25 traffic has to take the nearside lane and turn off here, it's the only junction like this on the M25. This not only saves mileage but is much easier, it avoids a somewhat convoluted roundabout at M25 Junction 3 where the M25 and M20 cross.

UK rental companies have no interest in International Driving Licences unless the original is unreadable (eg in Cyrillic or Arabic). So don't bother getting one. They see plenty of US licences each day.

If you are not used to a stick shift you will probably find this more troublesome than driving on the opposite side, and may want to specify an automatic, which are not that common at rental companies. I certainly would not advise renting a standard transmission without prior instruction if you have never driven one before.

On the 4-lane each way sections of the M25 you may like to keep out of lane 1 as, at many of the junctions, it turns off down to the roundabout. You will notice after a few such junctions the different lane lines for such lanes in the last half-mile before it goes off. Comparable to the "Right Lane Must Turn Right" situations in the US.