Basically persons and legal persons within the UK are not bound by international law. Only the High Contracting Parties, i.e., the signatories are bound to each other. So the UK could be liable to other states who signed it, but to within the UK the UN Declaration is an irrelevance.
Now, as Raymond points out, most of the important UN rights are also in the European Convention on Human Rights. The same thing applied (although there was a court system so some stuff actually got challenged by other states and, later, individuals) until the Human Rights Act 1998 was passed to give further effect to the ECHR. Now you can rely on your human rights against acts by public authorities and the courts will try to interpret existing laws to be ECHR-compatible.
Here, Article 11 ECHR is virtually identical to your Art 20:
Article 11
Freedom of assembly and association
1Everyone has the right to freedom of peaceful assembly and to freedom of association with others, including the right to form and to join trade unions for the protection of his interests.
2No restrictions shall be placed on the exercise of these rights other than such as are prescribed by law and are necessary in a democratic society in the interests of national security or public safety, for the prevention of disorder or crime, for the protection of health or morals or for the protection of the rights and freedoms of others. This Article shall not prevent the imposition of lawful restrictions on the exercise of these rights by members of the armed forces, of the police or of the administration of the State.
http://www.legislation.gov.uk/ukpga/1998/42/schedule/1
You will notice, however, that this particular right is qualified by the second paragraph. Some rights like Art. 3 - freedom from torture - are absolute. Others such as Art. 5 are conditional and may be broken in set circumstances (otherwise you could not imprison people for crimes following conviction by a competent court) and some like privacy, freedom of speech, assembly, religion etc. are highly qualified for obvious reasons, not least that freedom of expression (10) and privacy (8) directly oppose each other!
In this case, there must have been a legitimate reason (prevention of disorder), it was prescribed by law (Public Order Act) and the prevention of it must have been necessary and proportionate. The court will have taken HR into account when making the decision and there are even more stringent requirements for obtaining an injunction generally anyway.
