As a patient, I always have the option to go elsewhere and ask. (Which is one reason I would not go onto electronic prescriptions - as there were difficulties at one time.)
For reasons beyond my understanding, Boots almost always provide what I am happy with. Whereas most of the others don't. And I know in other areas, people get different makes even from Boots.
One make has the effect of causing stomach pain. And two other makes just seem slightly different and whilst it wouldn't be that awful to take them, I prefer not. The final make is new to the market and has never been offered to me.
The NHS Tariff prices are the same for all makes. What the pharmacy pays to its supplier, I have no idea.