Technically on the change between 2 systems I think voltage changeover is correct. Whilst for instance a changeover from HS1 to BR on the NLL curve both systems are nominally 25 kV the voltage between the 2 systems might actually be several kV difference but still within the tolerances for their different systems.
That's why the 24tph is described as theoretical
Switchover will be around 100mph as that is what the junction speed and approach will be.
But its not 24tph if you use just the headways. Its quite a bit higher?
The same would presumablly apply to moving between two lines fed from different feeder stations on the classic network.
I think the difference with HS1 and TGV is that while it is the same nominal voltage 25Kv the tolerances are different to the BR system. So for instance the low voltage trip has to be adjusted when changing betweeen systems. Im an Electrical Engineer but only an enthusiast in terms of the rail industry. There are a lot better qualified people than me on this forum to confirm.
Of course there was the instance in 1994 when 2 class 319s were sent into the tunnel a short distance. The particular units were fitted with Stone-Faiveley pantographs* which were standard on UK 25kV OLE. It was necessary to send the trains to Selhurst to have the overheight limit switches adjusted, but that was the extent of the temporary modification. It is probably likely that for continued high-speed (200+km/h) use that the design of pantograph would be designed for that but compliant with normal UK classic network speeds. Multiple pantograph types seem to be more common where trains traverse completely different systems, e.g. 25kV/15kV/3000kV etc. networks where lower voltages require higher upward forces.
The units actually ran all the way through the tunnel twice during 2013 on the handover from TML to Eurotunnel. It had to carry planks of wood for the VIPS to reach the platforms at Frethun.