When Britain's Territorial Force (TF) was reconstituted on 7 February 1920 after World War I, the former Essex and Suffolk Royal Garrison Artillery, which had defended the ports around Harwich, was split into separate sections.

The Suffolk Hvy Bde would man the Harwich batteries, the Kent and Sussex Hvy Bdes would combine as the Kent and Sussex to man the batteries from Dover to Newhaven, and a newly-formed Thames and Medway Heavy Brigade would be responsible for the Thames Estuary including the Medway defences.

[19][20] The manpower requirements for the forthcoming Allied invasion of Normandy (Operation Overlord) led to further reductions in coast defences in April 1944.

By this stage of the war many of the coast battery positions were manned by Home Guard detachments or in the hands of care and maintenance parties.

[8][20][24] By the end of 1944 serious naval attacks on the coast could be discounted and the WO began reorganising surplus coastal units into garrison infantry battalions.

[4][29][32][34][35][36][37][38] The following served as Honorary Colonel of the unit:[5] 415 Coast Rgt is believed to have worn an unofficial supplementary shoulder title with 'THAMES & MEDWAY' embroidered in the RA colours of red on navy blue.