Free things to do in London ?

Walk around and enjoy the city.

Nothing at the moment, we are in lockdown.

Nothing at the moment, we are in lockdown.

As you are not supposed to be leaving your home, not much.  You can exercise by walking through some of the countless green spaces and take advantage of walking through central areas without having to worry (so much) about the traffic.

Currently?
Enjoy the parks. Most other things are closed. 

Much will depend on what survives the lockdown & pandemic but check these out

suggest calling or email to see what their individual status' arehttps://www.freelondonevents.co.uk/https://www.timeout.com/londonhttps://www.londonforfree.net/

every Museum in London is free

I live there and there’re load of free things to do like exploring Canary Wharf’s roof garden or going to the parks