douglas, there is no limit on the amount of ibuprofen you can buy over the counter in a pharmacy; you can also buy 100 paracetamol and aspirin OTC in a pharmacy at the pharmacist's discretion.
Amounts are limited in supermarkets and other shops, even so, it is probably easier for most people to stock up on those in a single day simply by buying from different shops than ordering a prescription from the GP and collecting from the chemist.
My GP refuses to prescribe aspirin/paracetamol/ibuprofen that can be bought OTC - maybe the NHS Trust in my area won't allow it.