What would an A.B.C. mean, in London around 1920?

Question

I've just finished reading The Secret Adversary by Agatha Christie (freely/legally available to read online), and the term A.B.C. came up in a couple of places which puzzled me:

First of all, he must have a square meal. He had eaten nothing since midday yesterday. He turned into an A.B.C. shop and ordered eggs and bacon and coffee. Whilst he ate, he read a morning paper propped up in front of him.

“I took up a telegram to No. 891—the lady was there. She opened it and gave a gasp, and then she said, very jolly like: ‘Bring me up a Bradshaw, and an A.B.C., and look sharp, Henry.’ My name isn’t Henry, but——”

After a few muttered imprecations he handed the Bradshaw to Tommy as being more conversant with its mysteries. Tommy abandoned it in favour of an A.B.C.

I'm not sure if it means the same in all of these contexts (at least in the last two it probably does, being juxtaposed with Bradshaw which probably means Bradshaw's Guide), but I figured it would make sense to ask about all three usages together. What is "an A.B.C. shop", and what is an "A.B.C."? The setting of the story is London, quite soon after the First World War, so I'm going to say approximately 1920.

Answer 1

Yes, I believe that these are two different usages of the term "A.B.C.".

The first almost certainly refers to the Aerated Bread Company. Initially this was a company marketing bread that had been made without fermentation, just by simply injecting carbon dioxide into the dough, but as Wikipedia helpfully tells us "it is often remembered as running a large chain of tea rooms in Britain and other parts of the world", the tea room side of the business beginning in 1864.

The other A.B.C. is surely the ABC Rail Guide, a rival to Bradshaw's Railway Guide, which had the reputation of being easier to use.