In answer to a Freedom Of Information request from Paul Arvanitopoulos the Borough of Brentwood in Essex stated on 12th December 2017,  in respect to the section of the A12 running through their area:

The A12 was last litter picked in February 2015, prior to that it was done every 6 months over the past 5 years. Due to change in legislation we are currently reviewing risk assessments and staff training for signing and guarding requirements under the new legislation directions from HSE to ensure operatives are safe whilst working on more dangerous roads (i.e. fast roads or roads without pavements)

Read their response in full

The A12 is a Highways England Road but the local councils along its route are responsible for cleaning it.

See Cleansing of HE trunk roads (APTRS) – transfer of responsibility to HE

Peter Silverman
15th December 2017