[Petrochemicals (OP)]

No, the Roman Empire was mostly brought down by the "uncivilized" Visigoths who had colonized much of Western Europe,

That was rather the point in the op, Britian being a island, also with atrocious weather, even worse back then, with little development(Britian has been habituated and emptied many times due to ice ages). It must have taken much in resources to conquer Britian for little gain in manpower or riches, stopping a conquest of the East of the Rhine being captured and leaving Rome vulnerable. Would you reccomend invading a militant Canada if Mexico where a threat?

https://historypoints.org/index.php?page=britains-oldest-burial-site-gower

[alancalverd]

Britian being a island, also with atrocious weather, even worse back then,

Depends on your definition of atrocious. The east coast of Scotland was known in Roman times for its grape harvest and subsequent Vikings found the climate very conducive to invasion and agriculture. Roman soldiers seemed quite content with their short skirts and bare arms for the entire occupation and AFAIK there is little evidence of uniform greatcoats or fur skins, though they were common enough further north.  Chimneys and the use of indoor fires for space heating rather than just cooking, did not appear until the 11th century AD.