Haha, you do realise that we import more as a country than we export and that we export most to the EU, but still import even more from the EU than we export, don't you? And that, if Brexit does goes all 'no deal' the pound will continue its decline against the Euro. So if tariffs are tit for tat applied to each other, those tariffs will most hurt the party who's currency is declining and be of increasingly less pain to the party who's currency is ascending. Also, although its most marked with the EU, where our currency has fallen 25% in value over the past 5 years, the pound has also fallen against the dollar, the yang and the yen over the same 5 year period.
And a 4% fall (in a week ffs!) is a 4% fall whether or not the graph misses off the bottom 90%. Currency comparison graphs are normally written this way as typical daily fluctuations are usually smidgeons of smidgeons of a percent (and not a huge 4% in one week)