UK statistics in drug seizures made by the police or customs are published yearly by the Home Office.
Seizures
The number of drugs seized and people arrested vary greatly from year to year. This generally reflects the activity of the police and customs, rather than the amount of drugs available. See the Home Office website for the latest seizure statistics.
For example, a single find of a very large warehouse full of cannabis plants in a remote location might significantly push up the seizure statistics for the whole police force. This would not reflect any great increase in the quantity available, just the result of one very large find. Similarly, if the police in a particular area decide not to focus on cannabis possession, this will reduce the number and quantity of cannabis seized, without reflecting any change in availability. It could also, due to re-allocation of police time and resources, increase seizures of other drugs such as cocaine and heroin in this area.
Other drugs, such as LSD are difficult to detect, particularly if impregnated in paper while some are simply not seized by police. Seizure statistics do not cover solvents or the use of legal drugs such as some tranquillisers, magic mushrooms, GHB, amyl nitrites and ‘legal highsl.
Drug seizure statistics are helpful however in that they can, with care, indicate longer-term trends in drugs most commonly being imported and distributed in(to) the country. But they are not reliable on their own. A more reliable source of drug use levels is the Crime Survey for England and Wales.
What the statistics do show is that cannabis is consistently and by far the drug most seized, followed by cocaine, heroin, amphetamines and ecstasy.
Table 1. Police recorded crime by offence, England and Wales, 2011/12.
| Offence | Number of offences |
| 92A: Trafficking in controlled drugs | 29,939 |
| 92C: Other drug offences | 1,057 |
| 92D: Possession of controlled drugs (excluding cannabis) | 35,782 |
| 92E: Possession of controlled drugs (cannabis) | 150,959 |
| Total | 217,737 |
Drug purities
The seizure statistics reveal the average purities of drugs seized by police. These vary from area to area and also between drugs seized once they are in the country – and those seized at ports of entry by customs. This is because drugs already in circulation are cut to increase profits (particularly powder drugs such as cocaine and amphetamine).
Table 2. Average purities of drug seizures analysed by the Forensic Science Service, 2010/11
| Cocaine | Crack | Heroin | Amphetamines | |
| Police Forces | 28% | 32% | 30% | 9% |
| UK Border Authority | 64% | 62% | 42% | 49% |
Value of the drugs market in the UK
With these limitations in mind the Office for National Statistics (ONS) have tried to estimate the size of the drugs market in the UK, based on a number of assumptions. These include the number of drug users, the quantity of drugs consumed, street versus import purities (from police forensic tests) and the proportion of drugs seized at import. Using their best estimates, the ONS calculates that the value of the drugs market is between £3.9 and £8.5 billion a year.
Whether it is at the bottom or top end of this range depends on the customs seizure rate. If only five per cent of drug imports are halted, then illegal drug transactions add £8.6 billion to the UK Gross Domestic Product (GDP – how much we buy and sell in the UK). This is just over one per cent of the total GDP.