A man and a woman (his wife, one assumes) are sitting on a bench outside a country cottage. A Union Jack flag is flying from the roof. The man is reading a newspaper and there's an umbrella beside him (it's bound to rain soon, this is Britain!) Sheep are grazing in the adjoining field. The man asks, "Should we get out of the E.U.?". The woman replies, "We got in?"
I think that the cartoonist is highlighting the fact that, to a large extent, Britain already exists outside the E.U. It's not a member of the eurozone, and has had its 'special status' confirmed by David Cameron's E.U. renegotiation. The joke is that the woman doesn't realize that Britain joined the E.U. in the first place, or perhaps she's just being ironic. Either way, the message seems to be that Brits are not true Europeans.
Note the use of 'should' to ask for advice or an opinion. • Should I put my savings in an offshore bank account? • Should we be worried about the economic consequences of Brexit?