Green’s Dictionary of Slang

block of ice n.²

(UK und.) fraud.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 1: At the block of ice: welshing .
1938		F.D. Sharpe Sharpe of the Flying Squad 219: The professional welsher is a very different sort of person. The Shice or Block of Ice Mob as these gentry are known usually work in fairly large numbers and go out for pretty big money.