GTA London is exceptional within the series in two respects. It is the only Grand Theft Auto set in a real city under its actual name: not Liberty City or Vice City, but London. It is also the only game in the series that cannot be started on its own.
Both packs arrived in 1999, when GTA 2 was already approaching. London was therefore not a step forward but a branch: the same engine and rules in another time and culture. Contemporary reviews regarded that similarity to the original as a weakness. In retrospect, it is what makes the pack interesting, because it shows how versatile the original design could be.
London 1961 was also offered free of charge. That was far from routine in 1999 and is an early example of a publisher supplementing a paid expansion with additional free content.