Quite a long time ago I built myself a calculator
My worry was if I installed something which did not comply with volt drop requirements some time in the future some one could want me to correct it FOC so it is important to get it right, I was at that time regularly attending IET lecturers and the new edition allowed 5% instead of 4% and the lecturers said this equated to 106 meters of 2.5 mm² instead of from memory it was 88 meters, however in spite of using the correction factor Ct I could not get the figures quoted. So I asked, and was told for a ring final well the same would apply to a radial with 4 or 6 mm² we assume two sockets with a 20 amp load mid way for ring and end for radial and then the remaining is even throughout its length. And so we take the design current as 26 amp not 32 amp. And once I did that I got the figures they were quoting, since it uses square roots to work out the correction factor with the formula shown in BS 7671 it gets a little complex, not the sort of think you can work out on a fag packet, and my slide rule skills were never that good, so I used java script so I could use it with my phone.
However once made I realised if you allow +/- 0.02 Ω with both the origin and mid point measurement then it would need to be a long way out before one could prove it was too long when installed, so if the incomer was 0.35 Ω and the midway point was 0.94 Ω yes I could say it was within limits, but if I measured 1.02 Ω midway point I could not be sure that varying loads on the supply had not caused that reading, loop impedance meters may show to 2 d.p. but they rely on a steady supply to be accurate.
So before one could claim too much cable used you would need to have readings which were really well out, and although told 20 amp was what was used as centre load, if I consider it as 14 amp and 18 amp even spaced so Ib of 23 amp I get 123 meters, and 1.01 Ω and as you say nothing to say in the regulations I should not take 14 amp.
However the point is be it 14 or 20 amp, if using a 16 amp radial the current is closer to the limit for cable so the correction factor become closer to unity, plus still taking the full 14 or 16 amp at end so less of the load is divided by 2 as shared along the length, and in the main radials are 20 amp so in real terms you need three radials to cover the same as one ring final, not two, so there is a point in using radials, but at £25 per RCBO it means a radial system is around £100 more expensive to a double ring final system. And if all supplied from same RCD then what is the point, if one trips so do them all?