I have to agree with @BrianB.
But to understand the need for PfCO you really need to forget about photos/video - since this is not what the CAA really care about.
The CAA only care about why the flight is taking place - purely hobbiest or commercial.
And for commercial, as Brian also says, it is there being any intent at the time you take off that there will be a commercial/financial reason/reward/income.
Since the predominant reason for flying drones is for the taking of pics/video, this is also the predominant means to earn money from one - and probably the main one of interest to most members here.
But not the only one. Surveys, deliveries, bird-scaring (even at airports, ironically, Southampton use them).
You could be delivering flowers for someone to their mother, today - and if you’re getting a pint at the pub later for doing that, then technically you’d need PfCO to embark on the flight.
If you are in the construction/roofing business and you use a drone for survey work - even though there’s no money changing hands - it’s in support of the operation of a business and therefore also needs PcFO for the flight.
There have been several other threads that discuss all this in more detail. A quick search brings up the following.
To PfCO, or not to PfCO - that is the question!