The British-English expression Brylcreem Boy was defined as follows in It’s a Piece of Cake; or, R.A.F. ¹ Slang Made Easy (London: Sylvan Press, 1945), by Cyril Henry Ward-Jackson:

BRYLCREEM BOY. Fighter pilot (in the R.A.F.); one serving in the R.A.F. (Army). “Brylcreem,” most commonly used hair fixative in the Services, and synonymous with male glamour.

¹ R.A.F. stands for Royal Air Force, the name of the air force of the United Kingdom.

Cyril Henry Ward-Jackson rightly distinguished the use of the expression Brylcreem Boy within the R.A.F. from its use by the Army (i.e., the military land forces). The following two quotations indicate that, whereas the Army used Brylcreem Boy to designate any member of the R.A.F., the expression was applied to fighter pilots only within the R.A.F.:

1-: From Service Slang: A First Selection (London: Faber and Faber Limited, 1943), a dictionary of slang used in the British Army, Royal Navy and Royal Air Force, by John Leslie Hunt and Alan George Pringle—as quoted in The Sunday Post (Glasgow, Lanarkshire, Scotland) of Sunday 25^th July 1943:

Brylcreem Boys.—The R.A.F. (Army name).

2-: From Woman Pilot (London: Michael Joseph Ltd, 1957), in which the South-African pilot Dolores Theresa Moggridge (née Sorour – 1922-2004) recalled that, in her “comradely days of war-time in the A.T.A.” ²:

Fighter pilots sneered unmercifully at the transport pilots who replied with unkind references to ‘Brylcreem Boys.’

² A.T.A. stands for Air Transport Auxiliary, the name of an organisation having charge of the transfer of aircraft between factories, airfields, maintenance depots, etc., for the Royal Air Force and Fleet Air Arm during the Second World War.

The expression Brylcreem Boy originally referred to World-War-Two advertisements for Brylcreem featuring a fighter pilot ³. The following advertisement, for example, was published in The Bystander (London, England) of Wednesday 1^st November 1939:

BRYLCREEM
. . in the handy Active Service Tubes!
From all Chemists, Hairdressers and N.A.A.F.I. ⁴ in Active Service
1’- Tubes or Jars,
Larger bottles 1/6, 1/9, 2/6.
BRYLCREEM — THE PERFECT HAIR DRESSING

³ The model in those advertisements for Brylcreem was the British psychologist, anarchist and conscientious objector Hamilton Bertie ‘Tony’ Gibson (1914-2001).—Cf. the obituary of Hamilton Bertie Gibson, by Donald Rooum and Rufus Segar, published in The Guardian (London and Manchester, England) of Monday 30^th April 2001.
⁴ N.A.A.F.I. stands for Navy, Army, and Air Force Institutes, the name of an organisation running canteens and stores for service personnel in the British armed forces.

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

1-: From the poem used as the caption to the following drawing, published in Reveille: The Services’ Newspaper (London, England) of Monday 29^th September 1941:

We are not like the Brylcreem Boys,
We haven’t got a Press;
We’ve never raided Norway
Or got Musso in a mess.
We’ve no berets or topees,
We neither raid nor roam;
We’re those lucky “something” beggars
Who do soldiering at Home.

2 & 3-: From Country Diary By Suffolk Farmer, published in the Daily Herald (London, England):

2-: Of Friday 6^th March 1942:

Thanks for a Miracle

WHAT with the dogs barking, the cows blawing, and Sam peeking out of the loose box, it seems that something unusual is happening.
Even my aristocratic Essex saddleback pigs sit up and take notice. How smart they look just now!
They are getting ready for a special pedigree sale. Their white waistcoats have been washed with precious soap and dried off with sawdust. We have anointed their heads (and other black portions of their anatomy) with oil.
They are now so sleek and shiny that Mary the land-girl, whose brother is in the R A F, calls them the “brylcreem boys.”

3-: Of Friday 27^th March 1942:

These Pigs Make History

I DID not hear the chiffchaff this morning. But I did hear an even more pleasing sound, the cackling of a hidden hen, a clue that led me post-haste to the top of the haystack.
When, a little later, I descended the rickety ladder, it was with a hatful of brown eggs. And then yet another noise drowns the clamour of the Khaki Campbell ducks in the horse-pond.
A big lorry backs against the doors of the pig pens. Presently, we are all busy loading up my “brylcreem boys,” the pedigree pigs we are sending to market.
Not to an ordinary market, of course, or to a bacon factory. They belong to Society—the Essex Pig Society. Each is distinguished by a silvery ear-tab that carries his herd-book number.
What a schlemozzle! Blood-curling shrieks disturb even the complacency of Judy the Jersey, out in the meadow.
Sam lifts a pig by the tail, the boy George seizes an ear, and finally all these pig aristocrats are escorted into their first-class compartments.
They go to the Great Show and Sale. We wish them luck as we wave them good-bye.