In the UK more than 90% of the prawns used in the classic prawn sandwich are from coldwater prawns and the main supplying country has been Iceland. 

In a recent statement issued by Iceland's ministry of fisheries about the policies and management of Icelandic prawn fishing one can read: "Shrimp fishing is carried out using a shrimp trawl with small mesh, ie 36mm. For deepwater shrimp fishing (93% of all shrimp catching), fishing gear must be fitted with special equipment, a fish excluder (Nordmore grate), intended to skim off all fish out of the trawl while it is towed. 

This device is extremely effective and vessels pursuing deepwater shrimp end up with no fish in their trawls. The aim of the management of the shrimp fishing in Iceland is achieving sustainable utilisation of the shrimp stocks, ensuring that the fishing does not negatively impact other species, or over-exploit the shrimp stock itself." 

It is also worth noting that virtually no coldwater prawns are caught in British water and thus have no influence in the supply situation of prawns to the sandwich sector. This automatically nullifies yet another statement in the article that "a new prawn trawl designed by Seafish could help fishermen reduce the by-catch caught in their nets". 

This might be based on a confusion between prawns and nephrops (or scampi), caught off the coast of the north England. Up in Scotland and Ireland this scampi is often called prawn but would never enter a sandwich, much rather a breaded pub meal. 

Coldwater prawns used for sandwiches and salad dishes are probably one of the purest and most environmentally friendly wild caught seafoods.