The big issue with this is it almost assumes that all 27 countries have an equal distribution of imports and exports from/to the UK, when in reality it is likely to hit the big hitters (Germany, France, Spain) much more than it is going to hit Romania for example.
It's sort of using statistics to mislead IMO - yes 27 countries together have a much bigger economy that one country - but if 20 of those countries do very little trade with the UK then the GDP of those countries is just bulking up the numbers to suit an argument
Ok - lets take the two largest economies in the EU once/if the UK leaves - France and Germany.
In GBP/Sterling terms the sizes of the relevant economies are:
Germany - £3 trillion
France - £2.1 trillion
UK - £2.1 trillion.
The UKs imports and exports to those countries are:
Germany - Exports to UK - £78bn. Imports from UK £57bn
France - Exports to UK - £41bn. Imports from UK £40.5bn
So, on to relative dependence on bilateral trade:
Germany
2.6% of German GDP is dependent upon exporting to the UK
2.7% of UK GDP is dependent upon exporting to Germany
So the UK is very slightly more dependent upon trade with Germany than Germany is. In practice it is so close that lets say that we are both equally dependent upon this bilateral trade. Germany exports more to us than we do to them but it is in matched by how much larger the German economy is compared to that of the UK. You cannot say that Germany is more dependent upon UK trade than the UK is on German trade.
France
1.95% of French GDP is dependent upon exporting to the UK
1.9% of UK GDP is dependent upon exporting to France
Again, it is so close that lets say that we are both equally dependent upon this bilateral trade. In this case both economies are more or less the same size and export more or less the same amount to one another.
These balanced bilateral trading relationships would give us equal footing (in trade terms) if we were negotiating with France
or Germany, individually.
However, we are negotiating with a party that represents both. If there were disruption to UK-Franco-German trade, then we would suffer the impact on both our exports to Germany
and our exports to France, while each of those would only suffer the impact of the loss of trade to the UK. So in that sense we in the UK are roughly twice as dependent upon trade with Germany
and France as either of those parties are on trade with us.
Once you take into account all of the other EU countries we are dealing with, then the overall disparity of dependence of trade dependence becomes far greater.