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Most LU depots and sidings start from 21.
I'm sure it was done with future expansion in mind, ie not going below 1.
Ealing Common does start from A, B then 1, 2 etc.
Some, like Upminster, have recently added roads counting down from 21.
There was a thread in late 2013 that drifted onto this topic half way down the first page. Dstock7080 was also a contributor to it, there's a bit of history there about the reasons for the numbering schemes:
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