Boots
In larger Boots branches, there will be "display packs" of the "Pharmacy-only" meds (stuff like co-codamol and original, non-non-drowsy Benylin) on the shelves (basically an empty box in a plastic holder) which you take to the pharmacy counter, behind which all the good stuff is just out of reach.
In smaller branches, you will actually have to ask at the pharmacy counter. You can ask for Paramol (a more expensive brand of the same medicine, identified by a logo suspiciously reminiscent of a pinned-out eye) with a throwaway remark like "..... or if you do something of your own with the same ingredients but cheaper, that would do."
Be prepared for a few questions (are they for you -- yes, have you had them before -- yes, are you asthmatic or diabetic - no {obviously keep relevant meds handy at all times if you are}, don't take any other paracetamol-containing products with these, they might make you drowsy {you hope!}, don't take them for more than 3 days in a row {as if one packet would last you that long}) but it's hardly a full-on interview.