In the past, Trevor Teale used to organise monthly litter picking events in Coventry and advertise them and other useful information on a Facebook group.
Unfortunately this no longer exists.
You can now find your local group (or creating your own) by entering your postcode on the Litter Action whose website is https://litteraction.org.uk/
They also have a Facebook page at https://www.facebook.com/groups/litteraction/
You can also take part in the Great British September Clean 2020