As Cjd implies, images showing British cheques and receipts have disappeared from the OP.
The commercial overprint is for the London County and Westminster Bank Limited (1909-1923). Nowadays, it is NatWest.
The Commercial Overprint Society of Great Britain focuses on these type of overprints applied to keep staff from stealing stamps. The GPO would not buy back stamps that had a perfin or overprint. Here is their website
https://cosgb.blogspot.com/It has not been updated since 2014.
Some information also is available on the website of the GB Overprint Society:
https://www.gbos.org.uk//index.php/...t_Revenues/2