London simply can’t fully shut down it’s transit. People who can’t work from home (like, y’know, hospital workers) need it to get to work.

But likewise, it doesn’t make sense to run empty trains every 90 seconds that nobody is using, especially when losing vast sums of money due to vastly reduced ticket revenue. And there won’t be as many staff available (self isolation, parents looking after kids due to school closures, etc).

Demand will probably be down by 90% or more next week, so even if you reduce service by 50% you still have far less crowded trains than usual.