When Colonel Percy Granite invaded Mobius on New Year's Day 1999, he did so via a base in London. Sonic travelled there and disabled the dimension-crossing technology Granite used, then found himself wandering through Trafalgar Square. His reaction was one of horror, viewing this as an overcrowded and polluted "world gone mad!".
He went to 10 Downing Street to consult the Prime Minister for help against Granite.
The House of the Dead
By 2018, London (and the rest of the country) is a post-apocalypse ruin, overrun by zombies and monsters. Buckingham Palace has burnt down.
The sinister Resurrection Men have a base in Mayfair, the Grosvenor House Hotel; AMS raided it when retrieving a captive Sophie Rogan, taking a number of supplies and battling a horde of zombies in their escape. East of Eden and their Fear Foundry are in the Isle of Dogs, with Eden living in One Canada Tower. AMS and King "Hal"'s survivors are based in the City of London, commanded from the Tower of London. Other locations have since been visited or mentioned, such as Finsbury Circus, Savile Row, and the London Underground.
Midknight of the Eternal Champions lived in the City of London for 130 years, desperately attempting to find a cure that would stop him from being a vampire. Unfortunately, just before his major breakthrough, Midknight was killed.
In real world Earth, London was the site of the Sonic the Comic offices. They made their appearance in both Megadroid strips and Megadroid the Movie, although it was wrongly implied that the creative staff actually work there.
Segaworld, an entertainment complex often advertised and promoted in STC, was also in London, but has been replaced by a generic arcade.