Evening Michael
I take it you are on FTTH or FTTP; and how long have you had that?
As you state that the fibre is underground, how was this achieved?
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To try to answer your other questions, I generally assume that there is almost 1 fixed, traditional copper line per head of population, although I suspect that what-ever that Total Figure is/should be, about 25% to 35% are likely to be to non-domestic premises.
I have done a brief search for such a Total; but nothing jumps out.
This page seems to offer a clue-
http://www.ispreview.co.uk/review/top10.php
Table headed-
Top 10 UK ISPs By Subscriber Size
However, this is likely to include ADSL as it comes to approximately 24 million.
Back in January, BT OR reckoned that only about 15% of the lines in the Fibre "Coverage" areas, had been upgraded, mainly to FTTC.
More recently a later figure of 19% appeared; and both certainly tie in with the progress I have noted on "my" FTTC-
March 2014 Released for Service with 48 Filter Links fitted, out of a total back-plane for 288.
April 2015 an additional 48 Filter Links fitted, taking it to 96 present Maximum, out of 288 potential.
So just before that additional set was added, it looks as though the highest figure attributable of 48 of 288, amounts to 16.6 %, sitting nicely between the 13% and 19% OR figures.
I have not got absolute Line figures for "my" PCP; but by checking against various aspects, it appears to have approaching 300 lines.
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For FTTP and FTTH, assume that advantage is taken of the FTTC Fibre, leaving only about 300 "D-Side" lines to have Fibre added, to reach individual houses, premises etc.
Bearing in mind Andrew's comments on the fragility of fibre, and that it is very unlikely that any of those homes and premises have ducting suitable for blowing fibre through, it does suggest a massive task to convert "the whole country", particularly if that includes getting "D-Side" fibre to every flat in blocks of flats and similar.
It may be that running a fibre line over existing poles for several kilometres to a distant farm and such-like, could be a lot cheaper than to a house in the middle of a city.