I had a good chat with my new pharmacist and she explained how it works.
The cheapest are always issued but if you require a different one e.g. coated, you get your GP to state it on prescription.
If however you require the same make they can state the make on the prescription as well.
Only the GP can do this.
The different pharmacies may use different wholesalers so they would have to buy them in if it was a make they didn't normally have.
So it is up to us to make it clear to the GP's which we prefer.
I am a happy bunny now.