Green’s Dictionary of Slang

drunk; thus two tin hats, very drunk; three tin hats, incapably drunk.

1909		J. Ware Passing Eng. of the Victorian Era 246/2: Tin hat (Anglo-Port Said). Drunk – two tin hats very drunk – three, incapable, and to be carried on board.
1914		E. Pugh Cockney At Home 164: I got a bit tin-’at last night – done in the barrer-money – an’ now I’m gaspin’ for a wet.