London time explained
London runs on Greenwich Mean Time in winter and British Summer Time in the summer, moving forward an hour on the last Sunday in March and back on the last Sunday in October. It is the one major city whose winter time is also the world's reference point: UTC is anchored to the meridian that runs through Greenwich, a few miles east of the centre.
London does observe daylight saving, so its offset from UTC changes twice a year. The time above already accounts for whichever is currently in force.
Reaching London
The London working day overlaps the whole of Europe, most of Africa and the New York morning, which is a large part of why so many transatlantic calls get set for the London afternoon. Asia is the hard direction: Tokyo and Singapore have finished by the time London is properly awake.
How this page works
The time above is calculated on your own device from the IANA time zone
database that ships with your browser, using the zone
Europe/London. Nothing is fetched from a server, and the offset
shown is whatever is currently in force rather than a
number typed into the page — so when a country changes its rules, this
page changes with it.
The difference from your own clock is worked out the same way, comparing London's current offset against your device's. If your computer's clock is wrong, so is that comparison.