In the annual ‘Seizures of Drugs in England and Wales’ publication (which can be found here: https://www.gov.uk/government/collections/seizures-of-drugs-in-england-and-wales), the total amounts of cannabis, heroin, cocaine and ecstasy seized in each year are provided. However, no estimate has been made of the proportion of the total supply which such seizures represent.
The Home Office does not hold information on the total amount of drugs imported into the UK in each year. (ii) Estimates on the prevalence and frequency of drug use in England and Wales are published in in the annual ‘Drug Misuse’ publication (which can be found here: https://www.gov.uk/government/collections/drug-misuse-declared), but not on the total quantity of drugs consumed each year.
In a 2013 Home Office report ‘Understanding Organised Crime’ (which can be found here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/246390/horr73.pdf), the value of the illicit supply of various categories of drugs were estimated, which include heroin, cannabis, cocaine and ecstasy. The methodology can be found in Annex 2 of the report.