Yet another question for our former British overlords

[Sundae]

Even buying senna tablets I've been questioned as to why I need them. Give them the right reply and you get what you want.

Having worked for a team of pharmacists, I can only suggest the MIL gave an honest reply and the pharmacist did not feel it was an appropriate product.

Tell her to go to another pharmacy and lie, basically.
"I've had some temporary trouble with constipation and I find it very hard to swallow tablets."
Consenting adult, questioned, response given, job done.

There are certain products which while they are technically OTC, raise red flags for pharmacists. They can be struck off for malpractice far more easily than Doctors.