If supplies were not brought over from America before we entered the war the UK would have been invaded. Maybe if England was producing thier own small arms in large quantities there would have been less of our Merchant Seamen lost.
Germany's failure to knock out the RAF scuppered such invasion plans, The Battle of Britain forced a 'phony war' before the USA were involved.
What you mean to say is "Britain would have been forced into a peace treaty or ceasefire with Hitler, as Britain would have been starved had the USA not helped the largest merchant fleet (British) in the world bring food to England. Hitler didn't want war with Britain, and would have welcomed such a plan allowing him to fully concentrate on the East and his 'living space' plans."
The manufacture of guns for armed citizenry would made no difference, the luftwaffe were required to get control of the air and knock out the RAF.
Also there is no way to determine if large manufacture of arms within Britain would have saved American Seamen, Britain needed food, and other supplies. Unless American was happy letting German uboats run riot within it's coastal waters, by withdrawing all it's destroyers and escorts from the Atlantic, nothing would have changed and Amertican Merchants weren't directly involved until mid 41 long after the threat of invasion had passed.
