Any customer can express a preference, but the majority of pharmacies are sent what they are sent by their designated wholesaler.
I have to avoid Teva levothyroxine because of a known allergy to one its excipients. Levothyroxine is an essential medicine. A number of well-known pharmacy chains - all the nearest pharmacies to me, in fact - frequently only have Teva brand available, and can only say to me, 'Take it or leave it'. They can't order what their wholesaler doesn't stock.
In contrast, there are essential Teva medications, such as blood pressure meds, that don't contain the allergen, whereas other brands do.
That's the reality. Being on multiple medications with the now frequent swapping between brands and generics that pharmacies do - as approved by ICBs - is really draining for unwell customers/patients, especially during a time of medication shortages. Not everyone's able to repeatedly drive around from pharmacy to pharmacy seeking out multiple medicines and brands (and even then you might not be successful).
Expressing a preference doesn't supply essential medications to the patient. I would like to think that no-one vulnerable is feeling pressured to refuse Teva, although I understand the principles of boycott.
It may be more productive to analyse how and why Teva became the largest manufacturer of generic drugs in the world and why ICBs and pharmacies in England are so dependent on them. And would the boycott also apply to shareholders like Pfizer, including its epilepsy, oncology and antiparasitic drugs? It's a complex area. It's not carrots and Disney.