Over 48% of our worldwide export trade is with the 27 member states of the EU. Germany is by far the biggest member state in terms of trade and they had 7.5% of their exports with us. You need to look at the member states that have a trade surplus with us who have more fragile economies as possible allies. On that basis you would target Spain and maybe Poland, Czech Republic and Belgium to influence. About 20 member states including France have a brought parity of imports and exports give or take 10%.
Our biggest trade surplus with the rest of the world is with the USA where we import about £60 billion and export £100 billion. It will be interesting to see the favourable trade deal we negotiate with the Make America Great Again Trump Administration that greatly improves on tht position.