See the following report from NATS.
https://nats.aero/blog/2020/03/weath...affic-control/
And a quote from the page.
Visualisation showing 25 March 2019 – 6,224 flights – and 25 March 2020 – 1,415 flights, A 77% drop.
This is for March before the reduction in passenger traffic really kicked in. Would imagine April is going to be even more dramatic.