just a bit more info. I worked for a B.T. contractor, changing poles that had failed the test procedure because age had decayed them or they had been damaged etc. all the poles are dated, and sometimes we would pull out a pole from around the 1900s. some of these poles would be as solid and good as the day they were put in, the injected creosote having preserved them. we did find poles with knots and bits sticking out, these were usaually dated 1940-1945. probably during the war anything would suffice as a pole.
a few years ago a lot of poles from the baltics were imported, but the quality was not as good as the scottish ones and there was a safety issue, although this may have changed now.
alf