Salt Beef Sandwich at Selfridges Department Store

Why is it so difficult to get a salt beef sandwich in London?  The Brass Rail at Selfridges is always busy.  I have rarely been able to walk straight in, and to find there isn't a queue.

If it is so popular, why are there not more salt beef bars, cafes, restaurants on the London scene?

I am off to enjoy one right now.

Since writing the above, I spent two days doing the tourist spots with a couple of old friends.  This salt beef stall is at Borough Market

and as we walked from there back to Charing Cross, wandering through the backstreets of EC4, we came across quite a few salt beef cafes.