Well in "Northern Europe", the Bronze Age dosnt end before 500BC. So only a few centuries later (and the period 900-500BC is the Bronze riches period). And Bronze was still imported and used frequently for a few centuries.
I really doubt the "collapse" of Bronze trade theory for the introduction of Iron. As if it collapsed you would also see the effect on areas with no natural Bronze sources (neither tin or copper) such as Denmark. Also Iron was first used among the elite, then spread out, so the whatever qualities they were looking for they found iron preferable (perhaps weight?). If you dont have DnD fighters high carrying capacity an armour weight reduction of 10-20% might come in handy.
While Iron is in theory more plenty full than copper/tin, there is also the question of technology, knowledge and infrastructure: the people already knew how to work bronze, knew were the sources were and finally they already had the infrastructure organised around copper and tin.And yet here I am reading up on the Macedonian forces, and while they largely didn't use metal body armor per these sources, when they did, it was usually bronze ("muscle cuirras", greaves, helmet, some other possible pieces). Per same, there's indication from art and artifacts that they could make iron in large enough plates to make the same sorts of armor, but it would have been the province of the extremely elite or wealthy.
For example Denmark have many bog Iron sources, but the first certain evidence of them being exploited are 200BC, and the major peiod of extraction is 1-550AD (and then again in the 12-14th century AD). They first have to get a good working on the process, the fuel use etc.
Its true that we see a partial collapse of some of the copper mines, but in general I think Iron won in a fair competition.