you may have heard older people or history teachers make the claim that "we'd all be speaking german" if world war two had turned out differently, which, if i may intervene, is extremely unlikely. hitler had an easy enough time invading a couple of weak neighbours, but you have to realize that most of them actually didn't fight. the british and french didn't actually fight either, at least initially. it was called the phony war. 

the russians fought, because the alternative was being turned into soap for lebensraum. the nazis really planned to exterminate the russians, but they probably would have just absorbed the british.

as soon as the nazis faced a real army, they collapsed. as they were landlocked and didn't have colonies, they had difficulty with things like access to oil. they didn't stand a chance in the long war. the british empire, including america and australia, was simply superiorly positioned. it was just a process of fighting it out, which they tried to avoid because they knew it was harmful and dumb (and because they liked the idea of arming the nazis to fight the soviets).

yet, the truth is that, at least in the anglosphere, we already speak german because the brutal invasion of britain by german speakers that is being imagined hasn't just already happened, it's happened several times, from several angles - vikings, danes, saxons, angles, jutes, normans, etc. if you look at a map, anglia and saxony are regions in the german-speaking continent.

there's been some recent historical revisionism that suggests that the germanic speaking barbarians may have been invited into the region to overthrow the romans (there's a curious historical event called The Great Barbarian Conspiracy, which is recorded obscurely, but seems to be exactly that), who were seen by the british as a colonial invasion force, and that the celts may have willingly adopted a germanic identity for that reason, as the germans were the freemen (this is actually more like braveheart than marx) fighting the declining empire and winning. if you've been struggling against an unwanted colonial power for centuries and somebody starts winning, it's enticing to want to join them. that's probably the actual reason why we speak german - because the germans beat the romans (going back to the battle of teutoberg forest, which is a fun story to read if you don't know it, even if it's half legendary), and the celts got slaughtered by julius caesar in one of the worst genocides in human history.

it is a little unconventional, but, for this reason, i consider the uk to be a part of scandinavia.

if that hadn't happened, we'd all be speaking brythonic (brittanic, or british, which is what they used to speak in britain), breton, cornish and welsh.