So Mariupol has finally fallen.
Source? I don't see it anywhere. If you don't have one, then don't just throw that out there.
Now, if you meant
effectively fallen from a pure strategic point of view, then fair enough, but be more precise with your language.
I mean, Hitler dreamed of world domination which means eventually both Germany and Britain would clash. Arguably the smarter decision was to join the allies, rather than wait for a full strength reich to cross the Channel.
Besides, if you look at historical events, Britain always dreaded a unified Europe due to the danger it would posses, and specifically the idea of both german and france being under the same umbrella was anathema.
I'm not an expert on british history by any means, so I might be wrong on this/probably wrong, but I don't think the possibility of britain sitting it out while the nazis unified europe was ever a thing.
Lol, there never was the risk of a unified Europe then, and the UK didn't dread it.
Wildebeest
has laid that out in more detail.
To add to that, Hitler didn't seem (because this is alternative history) to want to invade Britain. He seemed to be open to an alliance, and it's not like there weren't quite a few sympathisers to that in the UK (many had spent the 20s and even 30s playing/messing around in Germany).
That's not to say the UK's choice to stand up to Nazi Germany was necessarily mainly altruistic (although, yes, the UK did see itself as a defender of democracy and freedom, and upholder of treaties). While a Nazi Europe (not really unified as there would have been the same old grudges still there) wouldn't have really been a threat to the UK herself, it would likely have ended up being one to the British Empire, and that was a concern.