It might be wise if Tim Worstall spent checking the assumptions here. Number one, it to take the weight of the copper, and not the whole cable.
This is 24 AWG cable with a core diameter of 0.51mm. That's 0.0255 x 0.0255 x PI x 100 = 0.204 cubic centimetres per metre length. Now double that for one pair and it's 0.408 cubic centimetres per metre length. Now add a (generous) 10% for the overhead of twisting (local network cable is lightly twisted). That yields 0.449 cubic centimetres per metre length of one pair. Multiply that by the density of copper (8.94gm per cubic centimetre) and that's 0.449 x 8.94 = 4gm of copper per metre of twisted pair. There are 1,609 metres in a mile, so that's 6.4Kg of copper per pair per mile. A 10 pair cable would therefore have 64Kg or copper per mile, not 132Kg per mile. That immediately slashes Mr. Worstall's valuation by over half as about half that weight will be sheathing.
Then we have the big question - is BT's figure of 75 million miles an estimate of the total length of cables, total length of pairs, or total length of wire? If we take the total number of BT lines in the UK it's of the order of 25 million. If that 75 million miles is 10 pair cables, that implies an average of 30 pair-miles per line. Even allowing for some spare pairs, this is clearly ridiculous. If the figure of 75 miles is pairs, then the total "pair length" per line is around 3 miles, probably not an unreasonable estimate - broad band spends indicate average line lengths are in the region of 2-2.5 miles leaving about 15-20% "spare" lines. If I do the same for total wire length, then the result (1.5 miles per line) is rather too low to be credible.
So, I would suggest that BT are quoting an estimate for pair lengths knocking 90% of Mr. Worstall's fanciful estimate.
So on that basis, we have 75 million miles of pairs x 6.4Kg = 480,000 tonnes of copper in BT's network. That's still £2.4bn of copper, a tidy sum, but nothing like the 20 x estimate that the author of this piece has calculated.
There's also another thing - if the value of the copper alone in BT's network was really £50bn, and we adopted a modest 7% ROCE, then Ofcom should be allowing a line rental income on that basis alone of £3.5bn per year, or about £12 per month on just the copper before including the capital value of the ducts, poles, maintenance, rates etc. As that's approaching half the wholesale monthly costs of a line, that's clearly nonsense.
So I would suggest, Time Worstall has overestimated the value of the copper in BT's network by about 95%.