Green’s Dictionary of Slang

implies approval of whatever is being considered, whether actual food or not.

1953		B. Behan Scarperer (1966) 90: ‘Another bit of bread till I rub up the last of the grease. Lovely grub,’ He munched juicily.
1966		P. Willmott Adolescent Boys of East London (1969) 44: We picked up some birds and took them to St James’s Park. Lovely grub!
1982		P. Bailey Eng. Madam 72: I pressed my tits against the blokes’ arms as I served them their meat and two veg. ‘Lovely grub,’ they said and they didn’t just mean the food.