Lived several years in London and Scotland.  All the major car rental companies rent (or hire as they say in the UK) rent cars.  Pretty much the same process as in the US.  You do not need an international driving license.

First time I drove a car in the UK was in central London.  Was young and foolish then.  It was an experience for sure.  I would suggest not doing that but go to agency in a suburb or another town - or the airport.  You do not want to rent a car to get around London.  They have very good public transportation.  The Tube is very easy to understand.  Buses are a little trickier when trying to get the right number bus going the right direction.  But a great way to see the city and locals. 

Driving on the left takes some getting used to and roundabouts are everywhere.  Watch out if you are in an area with no traffic.  It is real easy to turn into the wrong lane if you do not have traffic to follow.