"DAN GE," from the Thames foreshore. (Source)
Southwater brick. This red engineering brick originates from the brickworks in Southwater in Sussex, southern England, which made bricks from 1890 to 1981, when it closed and became a country park. Southwater bricks were used worldwide, but notably to build the London sewers and many London air raid shelters in World War Two.
Another brick. This appears to have been made by the Douglas Firebrick company, which had its works in Dalry, in the west of Scotland, from 1917 until closure in 1945: http://www.scotlandsplaces.gov.uk/record/rcahms/151621/dalry-douglas-fireclay-works-mine/rcahms?item=740580