We see Cameron walking along a street commenting on the poverty and deprivation. Of course, we are meant to think that he is already in India, where you more typically expect to find such things. However, there's a twist at the end. In fact, he's still in the UK!
The cartoonist is making the point that the economic and social situation in recession-hit austerity Britain has become so bad that it needs help (in the form of trade deals) from countries like India, which have traditionally received British aid.
Adams uses a similar idea for his cartoon in The Telegraph.