There's no reason why the escalators can't be from a different part of
the platforms to the current booking hall, is there?

The principle is OK but would depend on the distance between the platforms
and the street. ...

According to CULG the present lifts travel 34.3 m (112'5"). That's
significantly deeper than Angel, which may be another reason why Covent
Garden was never selected for conversion. If the whole distance is to
be covered by a single long flight of escalators, their ends would be
offset horizontally by 60 m or almost 200 feet. If the lifts are at
one end of the platform, the escalators could lead to the other end,
but if they're in the middle, that wouldn't work.

Of course, if this was the only problem it would always be possible
to avoid it by using two successive flights of escalators at different
angles, like at Westminster. That has advantages and disadvantages.
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Mark Brader | "The good news is that the Internet is dynamic.
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My text in this article is in the public domain.