Here's the relevant part of the Highway Code:
http://www.highwaycode.gov.uk/22.htm#217
Note that, unlike most of the other paragraphs on that page, there's no red 'Law' reference at the end of the paragraph. That means that the contents of that paragraph aren't actually embodied in statute law. However, the courts can consider breaches of the Highway Code rules when deciding whether a motorist has committed an offence. (In the days when we had 'proper' traffic wardens, rather than council parking officers, I've seen them using tape measures, in Clacton, to determine which cars were to be given tickets for 'obstruction' because they'd parked too close to a junction).
Chris