Yeah, I know. This 'law' is actually a guideline,
It is a law, not just a guideline - by law, all medicines must have a product licence stating how the medicine can be used and what ages it is suitable for, and it is against the law to sell a medicine outside of its product licence (i.e. to sell hydrocortisone for use on the face or to someone under 10 years old). The product licence for Medised (and the licences for all other behind the counter children's cough/cold medicines) now states that it must not be given to anyone under the age of 6 and so if I (as a healthcare advisor) sell Medised for a child under 6, I am breaking the law and the pharmacist on duty can be struck off.
Having said that, the product licence for Medised changed from 3 months+ to 2 years+ in March 2008, so even if the age hadn't changed to 6 this year, it would still have been illegal to sell it for a 17-month-old anyway.
However, medicine prescribed by the doctor is not subject to OTC licencing laws, so by getting it from the doctor, you know it's being prescribed safely, you're not lying about who it's for, the pharmacist is not breaking any licencing laws by selling it to you... and you get it for free.