The cartoonist portrays Cameron as a gravedigger who has just finished digging a grave in a graveyard, or cemetery. He's resting on a spade, and as he mops the sweat from his brow with a handkerchief, comments "Phew! Well, that's all the spade work done." A short ladder is positioned at the end of the hole, and a pick and shovel lie on the grass. Cameron is wearing green Wellington boots (or wellies) and has hung his jacket on one of the gravestones. A pile of freshly-dug earth can be seen to the left of the grave. Closer inspection reveals two worms emerging from the side of the grave with the heads of Cameron's cabinet colleagues Michael Gove and Boris Johnson, who is also mayor of London.
To fully understand the cartoon you have to know a couple of English idioms. Firstly, spadework (literally work done using a spade) is a metaphorical expression for difficult work that is done especially to prepare for something. • She had her assistants do the spadework in preparation for the trial. Here the reference is to Cameron's hard-won EU deal paving the way for the upcoming referendum. The cartoon is also meant to evoke the idiom to dig your own grave, which means to do something that will cause serious problems for you in the future. • By lying about the incident the minister has dug his own grave. Various interpretations are possible, but it could be argued that Cameron was digging his own grave by agreeing to an EU referendum in the first place, since if he loses and Britain leaves the EU, his political career will almost certainly be over. Now for the worms. Michael Gove has already said that he will support the 'Out' campaign in the referendum, and Boris Johnson is expected to do likewise. Peter Brookes' own tweet about this cartoon makes specific reference to the Shakespearean expression 'Even a worm will turn', which is used to convey the message that 'even the meekest or most docile of creatures will retaliate or get revenge if pushed too far'. (In the same tweet, the cartoonist says that things are 'looking grave for Dave', a pun on the word 'grave', which also means 'serious'). Cameron may now consider Gove and Johnson to be 'worms' (an insulting word for someone who is weak, dishonest, or insincere). We should also mention that Cameron was involved in a controversy last year over his choice of wellies (this is British politics!), so there may be a reference to that as well. Oh, and blue is the official colour of the Conservative Party (Cameron's shirt, trousers and jacket are all blue).
Phew! (pronounced 'few') is an expression used to show that you are relieved, tired, hot, or disgusted (here it's the first three).