I have had a similar concern, not so much to do with the business health of the prescribing companies (although good point, I am now!) but about what happens if there are shortages and need to switch to a different supplier as current one out of stock.
Generally, even if switching and they can see you were previously prescribed, they seem to not want to prescribe for patients newly coming to them with an under 30 BMI.
The only above-board solution I can see to this is to book an appointment with a private doctor, show them all the data and your progress and ask them to write a prescription so you can continue to get it from whichever pharmacy until within a normal BMI. This is how it seems to be done in the U.S., patients can then go to whichever pharmacy is in stock. This does of course come with an extra cost!
There is a slightly less above-board solution (not morally wrong though as it would just result in continuing the treatment as you would have if you had stuck with the same prescribing pharmacy). And, no, it's not sending in an old photo as they are time-stamped, but reluctant to mention it here!