The M25 starts (and ends) at the A282 - which is both a tunnel and a bridge. Jcn 1 is the south side of the Thames and proceeds in a clockwise direction to circle the capital. It ends at the A282 on the north side of the Thames at jcn 31.
London survived without an orbital motorway right into the 80s, by which time the M25 was being developed and built. Sadly, I believe it was far too little far too late. The M25 is hopelessly incapable of carrying the huge numbers of vehicles it was built to serve. Governments of the day (mainly the Tories during their years of dictatorship) have had a crack at trying to make the best of a bad thing, such as adding a few more lanes here and there, and fiddling with the speed limits on a time-of-day basis; but still, in the end, the M25 is a badly designed and badly thought out motorway.
Much of the problem is historical. This situation would never arise in the USA, possibly not even in Europe. In the UK, since the Romans built the first roads, all roads have been built on a town-to-town-to-village-to-town basis. If you examine any pre-1958 road atlas, you will see that there are no roads designed to take traffic away from towns. Consider the A1. Now, it is reasonably fast, and most of the towns it once passed through (Stamford B1081, Grantham B1174, Newark , Doncaster A638 etc) are now by-passed. But if you look where it used to go, and look at the renumbered sections through the towns, you can see the problem. Thus by the seventies, the simple A406/A205 North and South Circular Roads around the capital were becoming hopelessly choked with cars and goods vehicles who were not bound for London, but destinations to all points of the compass. Naturally, the residents of the suburbs through which these routes passed were peeved that all this "foreign" traffic was bunging up their roads. Initially, I think that the "London Box" plan would have appeared to be the answer to everyones prayers, but no! The demolition of so many homes and the very idea of living next to a motorway probably annoyed everyone even more. The "London Box" was scrapped, but the problem remained. The idea of an orbital motorway had been long talked of, but the task was so huge that it took ages to get it right, then to build it.
In the end, we got one circular route, when we could have done with three or four. Certain sections are now so busy, they have become no go areas, and we are rapidly going back to where we started, with drivers trying to find other routes around London to avoid the M25. Of particular nastiness is the M4-M40 section, which is crammed with traffic that has been there since the early nineties. The knock on effects of a choked motorway are that the routes that intersect then become crammed with vehicles trying to join. This is very dangerous, and many accidents are caused by traffic bunging up the inside lane of the motorway sometimes up to a couple of miles away from the jcn. This happens regularly around the M1 jcn, as I can testify, having been stuck there often, when I was intially on the M1, and only wanted to stay on the M1, but the masses of traffic from the M25 just slows everything down to a grinding halt. There are not many fans of the M25, and it's not difficult to see why.
There isn't much of interest on the M25, but that's a great shame considering you may be on it for quite some time. The new Thames Crossing is quite interesting. This came about due to the amount of traffic trying to get through the little tunnel. This was originally a dual carriageway tunnel, but now all four lanes are utilised in one direction (south to north) and the bridge takes the north to south traffic. As usual the amount of traffic using the M25 seemed to surprise the planners and as such most of the "improvements" have had to be implemented on the fly.
The fact that the M25 had to, as much as possible, utilise existing space, many roads were absorbed into it. For example, in the Watford area, the A405, an existing dual carriageway, was upgraded and provided some of the M25. On old atlases, you can still find the A405, from Watford to the bottom of the M10 (what a waste of a number) up to Hatfield. Also, the M16 which ran between the A1 near South Mimms to the A10 near Cheshunt became a section of the M25.
The M25 provides a through route for all traffic that is heading in any direction but wishing to avoid London, thus (theoretically) travel is possible from the M4 and M40 to the west to the M1, A1 and M11 heading roughly north. In a similar fashion, the M2, M20 which run away from London to the south east (serving Dover and Folkestone ports respectively), can connect with the above routes plus the M3 to the south west. Make sure that you travel at about 3am on Christmas Day to ensure the best road conditions.
So maybe I knock the M25 unfairly. On our crowded little island, the M25 is possibly the best we can do, considering the lack of space. We simply can't do what the US does, with their three figure freeway spurs and circulars. The M25 is not inherently a bad road, just that whoever originally planned it could not have forseen the volume of traffic that needed it. The section I used most was the dreaded M40-M4 section, given that I was the volunteer to go and pick up American Directors who were visiting our offices when I worked at Ford. Another bad thing about the M25 is that there simply is no way of avoiding it. In some cases, for example the A34, there is a perfectly serviceable route miles away from it. Having said that, there were still some diehards in the car delivery business who would still use the M25 when going to Southampton. Over to the east, there is no way of going from, say, the A1 or M11 to the M2 or M20 without using the M25. In my opinion, what London badly needs is another outer ring road, with another Thames crossing further east, perhaps near Southend. Of course, the amount of space required, the amount of houses demolished, and the volume of traffic flowing through previously rural areas will mean that this will never happen, but unless something is done, the M25 will become a huge traffic jam in both directions with constant repairs taking place. It's a nightmare scenario, but without extra road capacity, could well become reality.