That's an extension in one direction as opposed to the other - would exactly the same apply in the other direction?
Not quite true. The southernmost limit of 25kV is the southern end of City Thameslink platforms (i.e. atthe bottom of the ramp up to Blackfriars. The northernmost limit of the 750VDC 3rd rail is the northern end of Farringdon platforms. Between those points, on both tracks which are signalled for Bi-Di anyway, the dual powering is delivered through switchgear that apllies the appropriate earthing for whichever power supply a train passing along it is using. Thus a southbound train that couldn't swap to DC at Farringdon could continue under ac to City Thameslink. The reverse is also possible for northbound trains.
Could the DC be extended further north or ac further south, - well anything is possible but not necessarily desireable even ignoring the cost. Extending the DC on a 3rd rail system would increase the power supply requirements and involve additional feed points as the number of trains supported by the ECTS would exceed the maximum for a single feed when each train is rated at 3.3MW (8-car) or 5MW (12-car) particularly with the ruling gradients in the core being steeper than 1:30. In theory, it would be possible to take the DC to Kentish Town platforms and the Belle Isle portal of the Canal Tunnels as only dual voltage trains can use the core.
Southbound, the situation is different. As class 465 DC only trains operate into the Blackfriars bay platforms 3 & 4, it would be difficult to carry the OLE any further south thanplatforms 1 & 2 unless services north of Elephant and Castle were restricted to class 700s and mabe dual voltage Electrostars.
The problem for the line to London Bridge is that ac OLE must stop before the ramp joins the Charing Cross lines as although the northern pair is normally reserved for Thameslink core traffic, it might be necessary to send trains to/from Charing Cross for operational reasons. Most of the trains there are DC only. Dual powering those lines would be difficult as they are on a continuous viaduct were segregation of earths would probably be prone to signalling interaction despite immunity issues.
Be interesting to hear if any power lines come down with knock on effects for tomorrow's service with tonight's thunderstorm in full swing...
I think poor condition low cost MKIIb OLE used on the MML & ECML routes is much more prone to failure through use than from electric storms. High wind are of course an issue but the storm here now doesn'thave much of that.