Green’s Dictionary of Slang

squashed flies n.

also dead-fly cake, fly pies, squashed fly

biscuits containing currants, Garibaldi biscuits.

Farmer Public School Word-bk 85: Squashed flies, [...] biscuits with currants.
[UK]E. Pugh Cockney At Home 216: Two cups o’ sudden death and some o’ your very best dead-fly cake.
[UK]I. & P. Opie Lore and Lang. of Schoolchildren (1977) 184: Garibaldi biscuits are ‘squashed flies’ or ‘fly pies’.
[NZ] McGill Reed Dict. of N.Z. Sl.