An Aft-Cargo Smoke ECAM south of the I-O-W without further signs of smoke/fire entering the cabin is a no brainer... turn round and Mayday back to LGW. An overweight landing brings it's own problems but those are not insurmountable and certainly do not add to the current level of peril.
An Aft-Cargo smoke warning is not an automatic evacuation. The evacuation itself will cause injury and maybe broken bones for a few so is not a decision to be taken lightly. The average VS passenger might even get a few friction burns if they buy all their wardrobe at JD Sports
Once on the ground fire crews scan the aircraft surface with IR equipment for hot-spots within. Therefore a latched fire warning or cargo-smoke warning without sign of smoke in the cabin and without supporting hot-spot reports from the fire crew outside is almost certainly false. Any source of smoke in closed hold that has already been drenched in BCF is not an immediate threat and passengers can be disembarked via steps when they are available. Naturally the fire crews continue to monitor and evacuation remains an option at all times. The offending hold will not be opened until all the pax are disembarked.
It appears the crew were not presented with any evidence of fire onboard other than a single AFT Cargo Smoke.... Indeed to form a conclusion of imminent peril based solely on that would be faulted. To throw the passengers down the chutes is rather premature given those circumstances.
A big chuck-up to the CC however who got all the pax off safely when ordered to do so