A. 'The sewer system, along with the many many other tunnels beneath London, are not accessible to the public as this would be seriously dangerous. Any access has to be tightly controlled and monitored because of the overflow and diversion systems that Thames Water have in place. They are, however, incredible feats of Victorian engineering, often with decorative brickwork. The northern interceptor chamber looks like a cathedral!
As well as the sewers, there is the mail railway system, a small gauge railway designed to carry post east/west between sorting offices and mainline stations. This is disused. There are many tram tunnels in existence still too: the Holborn Kingsway tram tunnel being the most significant (partially used for traffic coming north off the bridge, the other part, going south from Kingsway, inaccessible but still contains the tram station). Camden own this and use it for storage and filming. You may get access by contacting their Highways Team.
There are also the subterranean chambers that carry all the electrics runs and data cables. These tend to follow main thoroughfares, the embankments and High Holborn for instance. Then there are nuclear war defences and bunkers associated with nuclear attack, mostly converted post-war from wartime bunkers. There is a huge amount of capacity. For example, beneath High Holborn there is a vast telephone exchange underground, built to withstand nuclear attack. Disused. Near Vauxhall there is a fire brigade control room, completely unexpressed at ground level except for an obelisk which looks like a memorial, but in fact is designed to split and deflect falling debris, to protect the underground chambers.'
A. :'There are some good books and web resources on the subject. Subterranea Britannica is the one that springs to mind.'
A. 'Sorry, the answer is no. But online you'll find accounts from intrepid explorers who've gone anyway. You may approach Thames Water or similar, but their answer will be a resounding 'No', I am sure. Hope this info is helpful at least.'