Well, maybe someone from the UK will get in on this, but still... I'm pretty certain you could find some codeine+antihistamine containing syrup someWHERE on your isle... A trip to another town's pharmacy might be in order, perhaps?
TBH though iono if they eve have otc codeine syrup in the UK, although that is probably what you had as a child (codeine+antihistamine I mean). But I can only speculate.
Can you get otc codeine pain meds where you live (I know from other and personal experience they're otc in the UK)? If you can't get those even, well, I don't know...
p.s. If you can you might want to change the identifier from "RC" to opioids or something like that. This isn't an RC issue.