\pounds is found in math mode, LaTeX executes
\mathsterling, otherwise it does
If we look at
newpxtext.sty in a TeX Live 2014 distribution, we find
The package is version 1.232, released 2015/04/07. This line is commented out in the current version 1.293 released 2015/08/07.
With your code compiled on a system using version 1.232 or earlier, the code points to the character in slot 163 of Computer Modern Roman (
cmr10), because this is the font used for operators; but the font has nothing beyond slot 127.
It's even worse if you also load
newpxmath, because you'll find
č instead of the sterling symbol, because the normal text font in T1 encoding is used and in slot 163 one finds č.
The line was a remnant of
pxfonts.sty, because the text font selected by this package indeed had a sterling symbol in that slot.
Preferred: upgrade your TeX system and always load
newpxmath along with
newpxmath and add manually to your preamble the current definition of
\mathsterling, that is,
To be honest, I don't find this new definition particularly appealing, because the symbol would not scale in subscripts/superscripts.
If you need an italic sterling in math mode, scaling in subscripts/superscripts, whatever is your TeX distribution, add
The output is the same using an older TeX distribution (I tested it with TL 2012, 2013 and 2014).