While Germany made its way into Poland and Czechoslovakia, still nothing

When Germany (and Russia) invaded Poland, Britain gave them an ultimatum to withdraw or war would be declared.

Just as Britain did over the invasion of Belgium in 1914.

Britain did not have to do that, and many voices were raised against.
Britain was still reeling and destitute from the previous war, and with every family still mourning someone who did not return from it.

they criminialised those who did by going off to fight in Spain
Not true.
Thousands went from Britain to fight the fascists and they were not "criminalised."