Is the COM terminal on the transformer (neutral?) at ground potential? That would be the UK approach however I gather that US/NA is different.
Nope. Here is NA, the 230 vac, both are hot leads. In NA, the pole pig in street has an UN-balanced 12.5 kv / 14.4 kv input ( one side grnded)..... and a BALANCED 240 vac output. In NA, the secondary of the pole pig has a grounded CT. That grounded CT is also looped back to the grounded side of the 12.5 / 14.4 kv. That's done for safety...in case of a 12.5 / 14.4 kv primary to secondary short.
There is 120 vac from either hot leg to the grounded CT. The CT is the neutral. In the main panel in the home, the Neutral buss and the ground buss are bonded inside the main panel. On any sub panel hanging off the main panel, the neutral and grnd buss are NOT bonded together. Reason for that is.... all fault current must flow through the ground wire.
In NA, BOTH sides of the 240 vac are hot, both must be fused (or a dual breaker, with tie-bar between em.... and both require a switch, like a DPST.
In the UK, and elsewhere, the pole pigs are UN balanced on BOTH the input HV side...AND the low 230 vac output side. In the UK, there is NO CT on the low V side. BUT, one side of the 230 vac is grounded / neutral. That grnded side of the 230 vac is also fed back to the grounded side of the HV input of the pole pig. Same deal, done for safety...if the HV side arced to the low V sec, inside the pole pig.
In the UK, only ONE side of the 230 vac requires a breaker / fuse / SPST switch.