Not entirely true.
Papaverum somniferum is legal to grow, the seeds and flowers and pods are legal to possess, but AS SOON AS THEY ARE "PREPARED" which by strict legal definition includes extraction of opium by any means, whether by laceration of pods, dissolving in water, whatever, an offence is committed.
As for codeine and dihydrocodeine in the UK; dosage units which are legal and available OTC are 8mg codeine phosphate/sulphate and what USED to be 7.46mg dihydrocodeine, now 10mg, as long as they are combined with a non-narcotic, the most common of course being paracetamol/acetaminophen. Combinations are available but only on Rx which contain up to 30mg of either. The most common brands of these are, respectively, SOLPADEINE and, for DHC, REMEDEINE (20/500mg) and REMEDEINE FORTE (30/500mg).
Codeine phosphate and dihydrocodeine tartrate may be prescribed without face-to-face consultation with a doctor, as in e-medicine, unlike any other with the exception of Oramorph solution, which is also NOT a Controlled Drug, as it does not contain enough morphine to be considered as such; like codeine and dihydrocodeine, it is classed as a POM (Prescription Only Medicine), also when in combination with Kaolin for the treatment of diarrhoea. Although I am not entirely certain whether that is an OTC medication, it certainly WAS ten or twelve years ago, but has fallen out of favour and now loperamide hydrochloride 2mg is the opioid of choice in that case.
Codeine phosphate is available pure in tablets of 15, 30 and 60mg.
Dihydrocodeine tartrate is available in tablets (SR) DHC Continus 60, 90 and 120mg.
It is also available in generic form of 30mg, DF118 Forte brand 40mg.
Under the Misuse of Drugs Act 1971 as amended, these substances as single drugs are classified Class B; almost all other opioids Are controlled drugs of Class A, Schedule 2 with the exception of tramadol hydrochloride which is Schedule 3.