The Highway Code
is clear cut on what constitutes legal or illegal but you have to read the first page.
Many of the rules in the 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 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 to establish liability. This includes rules which use advisory wording such as �should/should not� or �do/do not�.
http://www.direct.gov.uk/en/TravelAndTransport /Highwaycode/DG_070236
Reversing from a side road into a main road is an advisory 'do not'.