Green’s Dictionary of Slang

(UK school/university) congratulations.

1913		A. Lunn Harrovians 179: ‘Congrats, Cadby!’ ‘Damned glad you’ve got ’em at last!’ ‘Gratters, ol’ man!’.
1961		‘Frank Richards’ Billy Bunter at Butlins 2: ‘Gratters!’ said Johnny Bull.