I've had a search through the web pages of Defra and several other official bodies and organisations, but can't find any such rule regarding delays on motorways.
I did find, however, Council Regulation (EC) No. 1/2005 of 22 December 2004 on the protection of animals during transport and related operations and amending Directives 64/432/EEC an d93/119/EC and Regulation (EC) No 1255/97 - (pauses for breath!)
Article 23 thereof covers emergency measures where the transporter cannot comply with the Regulation and these include
inter alia:
Where there is no other means of safeguarding the welfare of the animals, they shall be humanely killed or euthanised So be careful what you wish for!
I'm sure that would be a last resort. If it was at all possible to allow the passage of a vehicle transporting livestock, and where not doing so would cause undue suffering, then hopefully that would be the course of action, however, what if, like ROG says, nothing could move? Where the carriageway is divided by a barrier that is capable of having a section removed I suppose it might be an option to allow the vehicle carrying livestock to join the opposite carriageway if one side is completely blocked.