The purpose of the BMCR is to build a database of factual information. For that data to have any value it must be verified. Cannabis remains illegal in Britain so there will always be some danger in contributing to any website or source of information, even if you do so anonymously or under a pseudonym.
After careful consideration the BMCR has concluded that the minimum requirement for data to be validated is a name, a part post code and a verifiable email address. The name and postcode cannot be verified so there is nothing to stop you using an alias.
Clearly, the information is only of any value if it is truthful. All data will be stored on encrypted servers and/or storage devices and will not be released to anyone voluntarily. However, you must decide for yourself the balance between providing information and your own security.