London has lots of datacentres in Slough (near Heathrow) and Docklands (London City Airport, but also many LHR planes transit over).

AWS/Azure/GCP/etc Colo in Equinix/Telehouse datacentres there, and those Docklands locations in particular were picked based on location to LINX (major London peering/interchange), and LINX was put in Docklands out of desire for relatively proximity to telcos, and London’s banking/finance industry.