As spring arrives and young leaves appear on hawthorn (Crataegus) children, who knew these as bread-and-cheese, would gather and eat them. Typically:
When we were young [in Buckinghamshire] we used to eat young hawthorn leaves; we called them bread-and-cheese [1].
In earlier times such leaves were eaten more widely, especially in time of hardship or famine. In May 1753 colliers at Kingswood, near Bristol, resorted to eating young hawthorn leaves as the marched on Bristol to protest about the export of wheat from the port [2].
Of course hawthorn leaves taste nothing like bread-and-cheese, but this name was given to a wide variety of mostly wild plants which were nibbled by children.
Species of Oxalis, which were widely eaten by children were usually thus called:
I was brought in Shropshire and Worcestershire in very rural settings … as children running wild all day we called wood sorrel [O. acetosella] bread-and-cheese and regularly snacked on it, though it tasted like lemons! [3].
Pink sorrel [O. articulata], native to temperate South America and cultivated as an ornamental, was eaten and known as bread-and-cheese on the Dorset/Somerset border [4], and in Suffolk, where it was known as pink shamrock [5].
During World War II bent grass [Agrostis] was eaten as bread-and-cheese by children in Elgin, Scotland, although ‘chewing it now, it doesn’t taste a bit like bread and cheese’ [6].
Other plants sharing the name include: common mallow [Malva sylvestris], the young seedheads of which were eaten, in Lincolnshire and Yorkshire [7]; common sorrel [Rumex acetosa], the leaves of which were eaten, in Devon [8], and silverweed [Potentilla anserina], the roots of which were eaten, in Somerset [9].

1. Natural History Museum, London, March 2011.
2. P. Lindegaard, The colliers’ tale – a Bristol incident of 1753, Journal of the Bath & Avon Family History Society, Spring 1978: 8.
3. anon., August 2011.
4. Milborne Port, Somerset, September 2011.
5. Leiston, Suffolk, July 2011.
6. Stevenage, Hertfordshire, January 1993.
7. J. Britten & R. Holland, A Dictionary of English Plant-names, 1886: 63.
8. C.H. Laycock, Thirty-ninth report on Devonshire verbal provincialisms, Rep. Trans. Devon. Ass., 58: 174, 1927.
9. A.S. Macmillan, Popular Names of Flowers, Fruits, etc., 1922: 37.

Note received from David Simms, May 2019:  ‘There is a Bread and Cheese Lane close to where I live in Cheshunt, Herts, which after research I found to be originally lined with hawthorn trees, hence its name.’

Images: main, wood sorrel, O.W. Thome, Flora von Deutschland, Osterreich und der Schweiz, 1885; inset, young hawthorn leaves, Museum of East Anglian Life, Stowmarket, Suffolk, March 2022.

Updated 16 March 2022.