Security in the UK in the 1920s

In the 1920s Russians were sending their agents in under the guise of communism. The ultimate aim for them is to destabilise democracy and instigate a coup or get communism to spread. And then in the 1930s you've got that threat from the fascists, from those that would support Nazi Germany. I suppose one could call them traitors in more time. So the security risk is not just behind enemy lines or across Europe in peacetime, we also have to look after security in our own country.