First of all only 16% of the population of the UK live in the Greater London area so most of us haven't a clue about it or even get involved with it because we don't go there. Some of the other big cities, Birmingham, Manchester etc have looked into it but as yet the juries out on whether it spreds around the country. The charge is only applied to the very centre of London so most of the capitals roads do not carry a charge to use, it is designed, so the authorities say, to reduce the volume of traffic in the city by getting folks to leave their car at home and use public transport thus preventing gridlock. However it is also a nice little earner for the local government as if you don't pay up if you enter the congestion charge zone, even inadvertantly , which is covered by CCTV monitoring with licence plate recognition, a nice little fine arrives in your letter box.
Apart from a few bridges around the country all other roads are free to drive on, oh with the exception of a relief section of motorway north of Birmingham called the M6 Toll Road, which is meant to take the pressure off the main M6 motorway through the West Midlands, it's a superb road but not used to capacity and truckers largely avoid it because of the charge so it has had little effect on the volume of traffic on the main motorway.
Don't know if you are any the wiser from that mate. Oh and you are out on the speed, the last time it was calculated the average speed during peak hours in central London was 8 mph and not much better in other cities, Birmingham where I live is is about 12 mph.
