Trains can't be guaranteed to be compatible based on the signalling system used. Clearance is geographically limited to where the train has been tested (or otherwise assured) to be compatible.How many different signalling systems in addition to the ones used in the Western region were tested on the 802s? Does the Southern region use the same signalling system as the Western region?
There are some notable differences in the way that signalling works on DC electrified lines, due to the high currents which flow through the rails.