It may have been applied in England. I don’t know whether the English post office was censoring incoming mail from Russia after WWI ended.
Or it may have been applied in Omsk by the British forces.
I am hoping someone knows.
Yes, it is true. However, a considerable number of troops was quartered and a British FPOs system created in North Russia. On the other part, as far as I know, there were in Omsk almost no or a negligible quantity of British troops with a few representative acting officers.