Think the headlines being bandied about are misleading. They all say you MUST return to work. What was actually said was people who are unable to work form home and needed to work can now return to work. Specifically mentioned food production, manufacturing and construction. London is not known as being a centre for food production or manufacturing and construction is by a long way not the main thing that people who work in London do. Those that are to return to work should do so from Wednesday - and there has not been enough time for companies to respond to yesterday's announcement and tell all their employees to show up today. As an aside the retention relief (furlough payment) scheme is to continue until further notice. So I suspect there is a lot of P-taking going on and we should not blame the government or assume that all those people are commuters
FWIW my company - which is in the industry for which London is best known - has said it will not open the London office till September at the earliest. This applies to most of our offices around the world (including the US) and the only people who will be allowed into the offices are those who have a business critical need to be on-site. This will be decided on a case by case basis and to be allowed into the office you would have to have your security pass re-enabled, which will only ever be done for a single day.