The 90 isn’t going to be powering the train - it is just a vehicle with an instrumented pantograph. The usual OHLE measuring vehicles (Mentor + the equivalent in the NMT) are our of action at present; so NR have improvised.
The 67s will power the train.
On the off topic point of whether the GW electrification is insulated from the rest of the network - there is no electrified connection to the rest of the railway from the GW at present. While there is a connection from North Pole depot to the West London Line, it is no longer electrified; and even when it was it was 750V DC 3rd rail, not 25kV AC OHLE.
The 67s will power the train.
On the off topic point of whether the GW electrification is insulated from the rest of the network - there is no electrified connection to the rest of the railway from the GW at present. While there is a connection from North Pole depot to the West London Line, it is no longer electrified; and even when it was it was 750V DC 3rd rail, not 25kV AC OHLE.