The share of Britain's exports going to the EU is declining, while the share going to the rest of the world is rising. If that trend continues, then in ten or twenty years' time, Brits will be asking themselves with even more urgency: Exactly what are we doing in the EU?
But the above chart underestimates the declining importance of the EU for Britain. The reason is that at the moment, British trade is clearly skewed towards the EU because of the tariff-free agreements that Britain has with the EU; i.e., if Britain had the same agreements with the rest of the world, then the share of its exports going to those non-EU countries would be even higher.
The chart is from here.