You are correct in that Wetherspoons have used trade deals negotiated by the EU to source cheaper products from non-EU countries.
It is also true that we will lose access to these trade agreements should we leave the EU without a deal.
As Ronnie/WB points put we can negotiate trade deals with any German supervision.
However, when negotiating any deal I would rather have the clout of a 508m potential customers rather than 66m customers.