The Easter Rising of 1916 was during the First World War, soldiers were sent to fight that one, even if it was small and not really long.

Also, the two islands are tactically important in a war (They are sitting right off the coast of the Island of Great Britain, after all) and the Shetland Islands has oil off shore, so there is value behind them defending it. And by attacking them, you became part of WW3 so you are part of the distraction for them, they have to focus on you too... And considering France was likely not trying to kill them (Probably too busy rushing to defend themselves from Germany, and although the EU collapsed I can't see the European countries declaring war with the UK (save for perhaps Spain in an attempt to reclaim Gibraltar)) you would become the nearest power and also the only power with a foothold in the UK, thus it's more important they deal with you first.
Given the resurgence of another militaristic German Reich, there were likely troops already rushed home with the express purpose of defending the British Isle against an invasion