In addition to those issued a warrant...
Police Officers without a warrant -
to search for drugs under the Misuse of Drugs Act 1971,
to search for firearms under the Firearms Act 1968,
to deal with a breach of the peace,
to arrest someone for an arrestable offence,
to recapture a person who has escaped from lawful custody
Customs Officers having a Writ of Assistance (an open-ended warrant which only expires 6 months after the issuing Monarch's reign ends) may enter any building or place at any time, whether by day or night, on any day, and search for, seize, and detain or remove anything liable to forfeiture under the customs and excise Acts. (Breathe easy though, if it is at night they must be accompanied by a constable.)