[..]
Even moving 300 miles down south will be a big change - I do not know the area at all so would have to find where the nicer places to live are [..]
They're not in Harlow.
As far as I can tell from your post, you want to take the job in Iceland but are worried because you see it as a big change and, crucially in this context, as a much bigger change than moving to Essex.
But is it really?
Iceland is a stable, civilised, developed country that's not very far from the UK and there is frequent travel available between the two countries at a reasonable cost. There's also complete modern communications between the two countries. Iceland and Essex are effectively about equally distant from where you live now. Not in terms of geography, but in terms of travel times and communiction (which is what matters on a practical basis).
The level of technology available is pretty much the same. You're not looking at going somewhere with uncertain water, sewerage, power, healthcare etc.
The weather is somewhat different in Iceland, but not staggeringly so in many areas (it
is staggeringly different further north, where temperatures as low as -35C have been recorded - whereabouts in Iceland is the job?) and in any case you know about it and the weather in Iceland appeals to you. The weather in Harlow would be somewhat different to what you're used to as well - it's significantly warmer and less windy in Essex than it is in northern England.
The only significant difference I see is language. There will be plenty of people in Iceland who can speak English (almost everyone, as far as I know), but obviously it's not the default language there.
I think that taking the job in Iceland would probably not be a bigger change, risk or inconvenience than taking the job in Harlow. That's based on the info in your post, since I don't know what the jobs are, whereabouts in Iceland the job is or what differences might exist between the companies involved. Maybe the one in Iceland is in trouble and might collapse leaving you without a job. Or maybe the one in Harlow is.