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Ichy.....the book is a "what if" story after all.
While I find it fairly easy to believe that we would have succumbed to the Germans if they had invaded Britain in Operation Sea Lion, especially as America had not come into the War at that stage, and showed no great willingness to do so, what I find less convincing is that organisations like the Met Police would have cooperated quite so enthusiastically.
Robert Harris and C J Sansom have both written alternate history novels similar to Deighton, and SS-GB is a cracking yarn, so perhaps we shouldn't be too critical.
After all, its miles better than Howards Way !