'Cock up' (also hyphenated as cock-up) is British slang for admitting an understated mistake on a fairly big order. It comes from military usage in the 1920s, based on the errors of either mis-cocking a gun or turning the brim of headwear up to resemble the cocked hat of an officer in the navy.
Above you will notice that I used the word ‘fairly’, beginning to succumb to the British tradition of diversion, subterfuge, and just plain sneakiness they all employ to both take responsibility for and deflect blame from any consequences. Most usages of ‘cock up’ deploy some modifier of this sort. There is even a minor meme, 'a cock-up on the X front' where X is used to remove the perpetrator from the person admitting their blunder. The phrase originated in the 1980s comedy series The Fall and Rise of Reginald Perrin, when a character said “there’s been a bit of a cock-up on the catering front”, on the occasion that there was no food because they hadn’t bought any. Note the words ‘a bit of’.
Someone involved in the insolvency of a business might say ‘I’m afraid there’s been a cock up on the accounting front’. An army official describing yet another event of ‘friendly fire’ might say ‘It seems they’ve cocked-up on the artillery front’. A mate at the pub whose round it is suddenly deciding to skip out might say ‘Oh dear, looks like there’s been a cock-up on the home front.’
And I, when the car has stalled on a lonely country road late one evening, just might have to say ‘Whoops, who’s cocked up on the petrol front?’