This is a work in progress, but I'll post as I go along to encourage input on what I may have missed. The intention is purely to consider the legal aspects of police spot checks, not the 'moral' aspects.
Search by police is governed by the Police and Criminal Evidence Act, and no doubt the Human Rights Act. Here then are what I consider to be relevant extracts from the PACE codes of practice.
Codes of practice – Code A Exercise by police of officers statutory powers of stop and search
2.9 An officer who has reasonable grounds for suspicion may detain the person concerned
in order to carry out a search. Before carrying out the search the officer may ask
questions about the person’s behaviour or presence in circumstances which gave
rise to the suspicion. As a result of questioning the detained person, the reasonable
grounds for suspicion necessary to detain that person may be confirmed or, because
of a satisfactory explanation, be dispelled. (See Notes 2 and 3.) Questioning may also
reveal reasonable grounds to suspect the possession of a different kind of unlawful
article from that originally suspected. Reasonable grounds for suspicion however
cannot be provided retrospectively by such questioning during a person’s detention or
by refusal to answer any questions asked.
(This is to do with stop and search powers outside of the home, but it does provide an important principle, that your refusal to cooperate will not of itself provide good reason to suspect.)
Codes of practice - Code B Searching premises and seizing property
1 Introduction 1.1 This Code of Practice deals with police powers to: search premises seize and retain property found on premises and persons
1.1A These powers may be used to find: property and material relating to a crime
1.3 The right to privacy and respect for personal property are key principles of the Human Rights Act 1998. Powers of entry, search and seizure should be fully and clearly justified before use because they may significantly interfere with the occupier’s privacy. Officers should consider if the necessary objectives can be met by less intrusive means.
2.3 This Code applies to searches of premises: (a) by police for the purposes of an investigation into an alleged offence, with the occupier's consent, other than: routine scene of crime searches; calls to a fire or burglary made by or on behalf of an occupier or searches following the activation of fire or burglar alarms or discovery of insecure premises; searches when paragraph 5.4 applies; bomb threat calls;
(d) subject to paragraph 2.6, under any other power given to police to enter premises with or without a search warrant for any purpose connected with the investigation into an alleged or suspected offence. (See Note 2B.)
2.5 This Code does not apply to the exercise of a statutory power to enter premises or to inspect goods, equipment or procedures if the exercise of that power is not dependent on the existence of grounds for suspecting that an offence may have been committed and the person exercising the power has no reasonable grounds for such suspicion.
(I am not aware of any such statutory power to enter and inspect premises that are not reliant upon suspicion of an offence, at least in relation to the subject at hand. These are more usually encountered in the likes of Trading Standards of Health and Safety legislation. Even then, decided cases appear to require compliance with Code B.)
5 Search with consent 5.1 Subject to paragraph 5.4, if it is proposed to search premises with the consent of a person entitled to grant entry the consent must, if practicable, be given in writing on the Notice of Powers and Rights before the search. The officer must make any necessary enquiries to be satisfied the person is in a position to give such consent. (See Notes 5A and 5B.)
5.2 Before seeking consent the officer in charge of the search shall state the purpose of the proposed search and its extent. This information must be as specific as possible, particularly regarding the articles or persons being sought and the parts of the premises to be searched. The person concerned must be clearly informed they are not obliged to consent, that any consent given can be withdrawn at any time, including before the search starts or while it is underway and anything seized may be produced in evidence. If at the time the person is not suspected of an offence, the officer shall say this when stating the purpose of the search.
5.3 An officer cannot enter and search or continue to search premises under paragraph 5.1 if consent is given under duress or withdrawn before the search is completed.
6.7 If an officer conducts a search to which this Code applies the officer shall, unless it is impracticable to do so, provide the occupier with a copy of a Notice in a standard format: (i) specifying if the search is made under warrant, with consent, or in the exercise of the powers described in paragraphs 4.1 to 4.3. Note: the notice format shall provide for authority or consent to be indicated (see paragraphs 4.3 and 5.1); (ii) summarising the extent of the powers of search and seizure conferred by PACE and other relevant legislation as appropriate; (iii) explaining the rights of the occupier and the owner of the property seized;
(These highlighted points presume that the visiting officer is conducting a search rather than some convenient (to the police) security inspection. Given that any alleged breach of security will automatically be considered an offence I believe it is a search with a view to discovering a possible offence. If you tell the officer that it is not convenient and he insists then I think that removes all doubt that it is a search and covered by the PACE codes of practice.)
To be continued......