What actually needs renewal on a 25kV AC system? I understand that some works have been required in areas (notably the Liverpool Street lines) where the original electrification was earlier and then 25kV AC was retrofitted. What has actually changed since the Blue Train electrification scheme? As far as I understand it the quality of overhead wiring went down since then, leading to the headspan situation on the ECML.
Also, even if work is required, it'll be of a different kind. All the infrastructure is already there to run an electric service - it's just about swapping out the bits which have become life-expired. There's no need for the major works required for totally new schemes, and legacy rights will exist to continue having lower clearances.
The main thing requiring renewal is contact wire. As pantographs pass over the contact wire, it wears down very slowly, but depending on speed of pantograph passage and number of individual pantograph passes, this can be quicker than if it was one pantograph at a lower speed.
The Anglia region is due to have its Mk1 era OLE renewed very soon, as large swathes of it operate with multiple pantographs (which obviously wear the contact down more quickly). Certainly on the GEML, speeds are also higher than most of the Strathclyde network, with 12-car (i.e. 3-unit) formations regularly operating at 100mph.
In the case of the renewals, the original 1940s/50s vintage OLE was
all fixed-tension, meaning wires would sag in the heat. Auto-tensioning was introduced with Mk1 equipment, which largely used copper for its cantilevers and wiring.
Mk2 made a move towards Galvanised Steel supports, but Mk3 had to be developed as, by the start of the 70s, copper prices had soared (hence the use of AWAC catenary in it).
Headspans meant that each structure could be prefabricated in jigs off-site, and rolled between masts during a T3(?) possession (with trains still passing underneath!). Their use was, in the main, driven by cost - HM Treasury had a fairly strong grip on BR's expenditure. Had Thatcher not come to power, we would have had a rolling programme of electrification (including wires to Penzance by 2000), but the rest, as they say, is history.
Another case for renewals is NR's "Campaign Changes"; replacement of unsafe parts, for example. Ceramic insulators (from Mk1 to Mk3) all have an inherent shattering risk if dropped, and can cause permanent eye damage to linesmen working on them or replacing them. Vandals can also break them by throwing stones/other projectiles at them, so "anti-vandal" (silicone rubber) insulators are installed whenever possible on existing OLE, and are installed as new for new schemes or large renewals (E.g. WCRM).
Balance Weights have fallen out of favour too, especially since the Potters Bar derailment; when the derailed 365 crashed, it hit a headspan with balance weights anchored there. Said weights then fell and crushed someone's head. As a result, Anti-Fall Balance Weights (with ratcheted wheels) are now used where possible. Spring Tensioners (Tensorexes) eliminate the risk of fallen weights entirely, and also make
independent tensioning of the wires easier. Otherwise, you'd need to rely on an
equalising plate at the anchor structure, which is difficult to maintain, alongside making higher contact wire tensions impossible.
