Section 19 of PACE, Police and Criminal Evidence Act, gives the police general powers to seize evidence.
Under section 19 when a constable (in uniform or otherwise) is lawfully on any premises he can seize anything which he finds on the premises if he has reasonable grounds for believing:
(i) that it has been obtained in consequence of the commission of an offence; or
(ii) that it is evidence in relation to an offence which he is investigating or any other offence;
And (iii) that it is necessary to seize it in order to prevent it being concealed, lost, damaged, altered or destroyed.