Why not do the changeover just west of Worting Junction?
I think I see where you're going there and there are some benefits:
Having an automated dual electrified changeover area at speed away from any major stations and junctions avoids complex OHLE, and dual immunity and bonding issues for track circuits through pointwork.
Any DC leakage current encouraged by the dual system area likely to be less of an problem in a more rural area between settlements with sparse undergound utilities.
From what I gather, only the Reading bay at Basingstoke would need AC electrification, so it would infact be quite easy to keep AC and DC isolated.
Except the whole point of electric spine is to allow electrified freight to run through on electric traction all the way from Southampton to the north. Even if the new freight locos were dual system AC/DC (highly recommended by the way for general flexibility and working on DC through Southampton!) there would need to be a short section of DC as well as the AC on the Reading branch or some additional AC through the mainline platforms at Basingstoke for these trains to changeover back to AC on the move after having changed over to DC near Worting Jn for the short glide through the western approaches to Basingstoke. Switching systems twice like that in such a short distance for the freights and other AC/DC trains on this axis (e.g. new electric Cross Country passenger trains) would be undesirable, although leaving the Basingstoke station area DC for a while could be acceptable as a useful project phasing tactic, allowing easier rural knitting to go up in advance of the more complicated station areas and be made use of straight away by dual fitted trains. This is a bit like Hitachi bi-modes in the west country. As long as the changeover process is quick and reliable any new bit of incremental GW AC electrification further towards the west is useful to improve performance and emit less noise, diesel soot and CO2. Even early isolated sections of AC such as over the steep South Devon banks west of Newton Abbot could be useful on that route.
Although generally undesirable, dual-electrified track must be accommodated at key places like Basingstoke (proposed) or Ashford (existing) for changeover purposes either at speed or whilst stationary. It is unavoidable. The length of the dual-electrified sections should be kept to a minimum, however, to limit the zone around which greater stray DC current leakage will occur due to the systems' conflicting return current and safety earthing philosophies. On the other hand a changeover zone should not be so short that a train failing to changeover successfully on the move will become stranded on the new system.
It is possible today to use axle counters for train detection throughout complex junction and station areas, allowing all insulated rail joints and impedance bonds to be removed and permitting a simple yet massively parallel cabling scheme joining all rails to each other frequently and providing an exceptionally low return resistance path to discourage all but the most hardy of return current electron charges from taking the stray path. Additional heavy parallel strengthening conductors can be added to help keep resistance reliably low. Even with these measures some issues with stray current and resultant corrosion should be expected and monitored for. Metal structures in the area should be assessed and designed with the issue in mind.
At Basingstoke, the final dual system should probably incorporate the entire station area (except the Reading branch bay which can be AC only), extending on approach along the main line in each direction sufficiently far back to allow an emergency stop at the station for a fast train that attempts to change systems on the move but fails. At the station, further attempts to changeover at rest may be made, but if still unsuccessful the train can be cancelled and safely reversed back into the electrification territory it came from or placed in a local siding for later action.
