London is the hub of the UK's road

London is the hub of the UK's road network and is easy to reach by car, even if driving into the centre of the city is definitely not recommended. Greater London is encircled by the M25 orbital motorway, from which nearly all the major trunk routes to Scotland, Wales and the rest of England radiate. The most important are listed below.

M1: The main route to/from the North, leading from the East Midlands, Yorkshire and terminating at Leeds. Most importantly, Britain's longest motorway - the M6, branches from the M1 at Rugby, leading to Birmingham, Liverpool, Manchester, the Lake District and onwards to the Scottish border, and ultimately Glasgow.
A1/A1(M) The A1 is the original, historic "Great North Road" between England and Scotland's capital cities and has largely been converted to motorway standard; it runs up the eastern side of Great Britain through Peterborough, York, Newcastle and continues north through Northumberland and the Scottish Borders to Edinburgh.
M40/A40: Arrives in London from a north westerly direction, linking the city with Oxford and providing an additional link from Birmingham.
M4: The principal route to/from the West - leading to Bath, Bristol and cities South Wales (Cardiff and Swansea). It is also the main route towards Heathrow Airport.
M3: The main route to London from the shipping port of Southampton.
M2/M20: Together, these motorways are the main link to the coastal ferry (and Channel Tunnel) ports of Dover and Folkestone from Continental Europe.
M11: The M11 connects Stansted Airport and Cambridge to London, and it terminates on the north eastern periphery of the city.
In addition to the M25, here are two inner ring roads in London which skirt the central area:

A406/A205 North Circular/South Circular The North Circular is a half circle on the North of the Thames, and is mostly a dual carriageway. It has direct connections with the M4, M40, M1 and M11 motorways and can be useful if you want to quickly get around the northern suburbs of the city. The corresponding South Circular is really a local road which is made up of segments of main suburban thoroughfares. The two roads are connected at the east end of the circle in North Woolwich/Woolwich Arsenal by the Woolwich Free Ferry, which runs approx. every 10–15 minutes and is free of charge, although it can only carry a limited amount of vehicles so avoid during busy periods as the queues can be very long! The ferry stops running after 10PM, so it's advisable to travel through the Docklands and use the Blackwall Tunnel instead.
Comparatively few people will actually drive into (or anywhere near) the centre of London. The infamous M25 ring road did not earn its irreverent nicknames "The Road To Hell" and "Britain's biggest car park" for nothing. The road is heavily congested at most times of the day, and is littered with automatically variable speed limits which are enforced with speed cameras. Despite the controversial "congestion charge", driving a car anywhere near the centre of London remains a nightmare with crowded roads, impatient drivers and extortionate parking charges (that's if you can find a space in the first place, that is!). From Monday through Friday, parking in the City of London is free after 18:30; after 13:30 on Saturday and all day Sunday.

There are also a number of Pay as you go car rental companies operating around London including WhizzGo and Car Clubs