Hang on – detaining someone without trial is illegal? Well who’d have thought it?

Article 11
1. Everyone charged with a penal offence has the right to be presumed innocent until proved guilty according to law in a public trial at which he has had all the guarantees necessary for his defence.

(The United Nations’ Universal Declaration of Human Rights to which, the last time I checked, the UK remains a signatory)

Alternatively (.pdf),

Article 6 – Right to Liberty and Security
3. Everyone arrested or detained in accordance with the provisions of paragraph 1.c of this article shall be brought promptly before a judge or other officer authorised by law to exercise judicial power and shall be entitled to trial within a reasonable time or to release pending trial.

(The Council of Europe’s Convention for the Protection of Human Rights and Fundamental Freedoms, to which the UK was also, last time I checked, a signatory)