The GWR was the only one of the Big Four who used solid cast brass numberplates, everyone else just painted them on. Therefore it was sensible to leave the GWR numbers, generally 4-digits, as they were, and renumber the rest.
Diesels (very few) were all renumbered into the 1xxxx series, and electrics into the 2xxxx series nationally.
Southern had 30000 added, together with the newest locos from Bulleid's bizarre numbering scheme being brought into this series.
LMS had 40000 added. Their main series with the newest LMS-built locomotives was under 9999, so became 4xxxx, but they had a lot of 10000+, so these became 5xxxx, and actually quite a few of the oldest 20000+, from their own previous renumberings, these were renumbered again mainly in the 58xxx series, to fit in.
LNER had 60000 added. The LNER had done a very logical renumbering just a couple of years before nationalisation so this logic continued.
Numbers 70000 and upwards were initially unused, they later got used for BR standard locos in a supposedly logical scheme which was very wasteful of the numbers.