In military slang, the expression Spam medal (also Spam Medal and spam medal) denotes a medal awarded to all members of a force—especially the 1939-1945 Star, awarded to British service personnel who took part in the Second World War.

This expression refers to the ubiquitousness of Spam [note 1] as a foodstuff during the Second World War, and is therefore comparable to the British-Army expression rooty gong, attested in 1925 and denoting a medal for long service and good conduct: the noun rooty denotes bread, and the expression rooty gong likens this medal to a regular ration issue, like bread.
—Cf. also the phrase to come up with the rations, meaning (of military medals and decorations): to be awarded automatically, without regard to merit.

According to Wilfred Granville (fl. 1952-62) in A Dictionary of Sailors’ Slang (London: Andre Deutsch, 1962), in the case of the 1939-1945 Star, the expression Spam medal additionally alludes to resemblance of the colours of the ribbon to those of the armbands of waitresses in NAAFI [note 2] canteens—as quoted in the Oxford English Dictionary (online edition, March 2022):

Spam medal, 1939-45 star whose ribbon has the same colours as the NAAFI girls’ arm flash. As spam, a kind of spiced-ham, was sold in the NAAFI canteen, what more obvious term could suggest itself?

The earliest occurrences of the expression Spam medal that I have found are as follows, in chronological order:

1-: From the column Town Crier, by Anthony Weitzel, published in The Detroit Free Press (Detroit, Michigan, USA) of Monday 31^st January 1944:

Ensign Tom Kent writes from England that the best GI humor of the year concerns that green, brown, black and white campaign bar worn by men who have seen service in the Afro-European theater of war. The Army boys in England call it, rather diffidently, “the SPAM MEDAL.”

2-: From the column The Town Crier, by Kenneth Nichols, published in the Akron Beacon Journal (Akron, Ohio, USA) of Friday 11^th February 1944:

Returning soldiers sporting the Afro-European campaign ribbon refer to them as the “Spam” medal.

3-: From the interview of “Maj. Joe A. Brown of Gillespie, Ill., a lead bombardier now in Council Bluffs”, published in the Council Bluffs Nonpareil (Council Bluffs, Iowa, USA) of Friday 26^th May 1944:

The DFC ribbon he wears, with a gold cluster, standing for 6 additional awards of the medal, was won “just for excellent results in bombing.” Nothing exciting he indicated. […]
[…]
Maj. Brown also wears the air medal with six gold clusters, the purple heart, the American theater ribbons for submarine patrol off the Florida coast before going overseas, the American defense ribbon and the European theater ribbon.
The latter, he said, we call the Spam medal, because that is our chief food in England. If you win a cluster (he wears a gold cluster signifying 6 additional major missions) the boys in England say you have “Spam with lettuce.”

4-: From The Wells Journal and Somerset and West of England Advertiser (Wells, Somerset, England) of Friday 21^st July 1944:

Mrs. F. I. Collins, secretary of the Clareville Social Club, has received a letter from Commander Piggot, of H.M.S. Fleetwood, the City’s adopted ship, saying that they have just received the parcel of good things from the Club members.
The Commander writes: “We much appreciate your kindness and thought for us. The comforts have been distributed amongst the men, who found them very welcome, particularly the reading matter.”
[…]
He also adds “You’ll be pleased to hear that in our last little “do” with the enemy, our Captain was awarded a bar to his D.S.O. His chest is a veritable blaze of colour now, with his O.B.E., double D.S.C., and “Spam medal and Clasp.” This latter medal is a popular name for the 1939-43 Star.

5-: From Central Illinois Men And Women In The Armed Services, published in the Illinois State Journal and Register (Springfield, Illinois, USA) of Sunday 23^rd July 1944:

Corp. Gene Fortner, son of Mr. and Mrs. J. J. Fortner, 322 West Calhoun avenue, is serving in the supply department of a field artillery unit in Normandy […]. Fortner has been awarded the Good Conduct Medal fondly termed the “Spam” Medal by the soldiers—who have the trade named product as an item of diet regularly.

Notes:
1 Spam (trademark): a tinned meat product made mainly from ham. There is an interesting parallel between the fact that Spam medal refers to the ubiquitousness of Spam as a foodstuff and the fact that spam (in the sense of unsolicited messages sent out over the Internet) appears to derive from a sketch by the British Monty Python comedy group, set in a cafe in which every item on the menu includes Spam.
2 NAAFI (acronym, from the initial letters of the Navy, Army, and Air Force Institutes): an organisation which has run canteens and stores for service personnel in the British armed forces since 1921.