They look to me as if they have very porous surfaces. I would guess that they are detector finds that have been dug up and cleaned.
The British pennies and halfpennies up to 1958 were struck from 95.5% copper, 3% tin, and 1.5% zinc. Afterwards they were struck in 97% copper, 0.5% tin, and 2.5% zinc (this alloy had previously been used during WW2). The 97% copper coins with the reduced amount of tin corrode badly in some soils and it may well be that a detectorist found these coins and cleaned them to aid identification - they were of no real collector value whether cleaned or not.