Here's an idea - look it up!
Highway Code
https://www.gov.uk/rules-for-cyclists-59-to-82
66
You should
- never ride more than two abreast, and ride in single file on narrow or busy roads and when riding round bends
The introduction to the Highway Code states:
Many of the rules in The Highway Code are legal requirements, and if you disobey these rules you are committing a criminal offence. You may be fined, given penalty points on your licence or be disqualified from driving. In the most serious cases you may be sent to prison. Such rules are identified by the use of the words
‘MUST/MUST NOT’. In addition, the rule includes an abbreviated reference to the legislation which creates the offence.
Although failure to comply with the other rules of The Highway Code will not, in itself, cause a person to be prosecuted, The Highway Code may be used in evidence in any court proceedings under the Traffic Acts (see
The road user and the law) to establish liability. This includes rules which use advisory wording such as ‘should/should not’ or ‘do/do not’.
So, it's not backed up by MUST or a reference to specific legislation, so is not law but advisory.