There will be prevailing road traffic conditions (carriage size, visibility, weather, amount of other road traffic etc) where cycling two abreast may amount to Dangerous, Careless or Inconsiderate Cycling as defined by Sections 28 & 29 of the Road Traffic Act 1988, and in those circumstances you run the risk of being reported for summons, depending on how the officer interprets the situation. Common sense generally prevents people from rendering themselves liable to prosecution.
Regarding the Highway Code – Section 38 (sub section 7) of the Road Traffic Act 1988 states –
A failure on the part of a person to observe a provision of the Highway Code shall not of itself render that person liable to criminal proceedings of any kind but any such failure may in any proceedings (whether civil or criminal, and including proceedings for an offence under the Traffic Acts, the M1Public Passenger Vehicles Act 1981 or sections 18 to 23 of the M2Transport Act 1985) be relied upon by any party to the proceedings as tending to establish or negative any liability which is in question in those proceedings.
… so, if the HC says don’t do it, you ought not to – as it could be quoted to lend weight to a prosecution case!