This question is very scenario-dependent. I will outline the answer relating to stop and search below. If what is set out below is not complied with, then the stop and search is not lawful and you are neither obliged to stop, nor consent to be searched.
The law in england-and-wales regarding when individuals must stop and comply with the request of a police officer for stop and search is set out in the Police and Criminal Evidence Act 1984 ("PACE").
Location
You are only required to stop and comply if you are in:
- somewhere the public has access to (s.1(1)(a) PACE), or
- in any non-dwelling private land where people have ready access (s.1(1)(b) PACE)
In general, you do not have to comply if you are in:
- a private dwelling (s.1(4), s.1(5) PACE) (with some exceptions)
Justification
The police must have reasonable grounds for their desire to stop and search you. There are two recognised grounds for this:
- To find stolen or prohibited articles (s 1 PACE)
- To find controlled drugs (s 23 of the Misuse of Drugs Act 1971)
The officer's suspicion must be both genuine (subjective requirement) and reasonable (objective requirement).
Duty to inform
From Paragraph 2.11 from the PACE Code A 2023 (my emphasis in bold):
2.11 There is no power to stop or detain a person in order to find grounds for a search. Police officers have many encounters with members of the public which do not involve detaining people against their will and do not require any statutory power for an officer to speak to a person (see paragraph 4.12 and Note 1). However, if reasonable grounds for suspicion emerge during such an encounter, the officer may detain the person to search them, even though no grounds existed when the encounter began. As soon as detention begins, and before searching, the officer must inform the person that they are being detained for the purpose of a search and take action in accordance with paragraphs 3.8 to 3.11 under “Steps to be taken prior to a search”.
More generally, even if you are not detained, paragraph 3.8 deals explains that the officer must take reasonable steps to inform:
3.8 Before any search of a detained person or attended vehicle takes place the officer must take reasonable steps, if not in uniform (see paragraph 3.9), to show their warrant card to the person to be searched or in charge of the vehicle to be searched and whether or not in uniform, to give that person the following information:
(a) that they are being detained for the purposes of a search;
(b) the officer’s name ... and the name of the police station to which the officer is attached;
(c) the legal search power which is being exercised, and
(d) a clear explanation of:
(i) the object of the search in terms of the article or articles for which there is a power to search; and ...
...
Conclusion: The bottom line is that a police officer cannot impose a legal duty on you to engage without reasonable grounds to do so. However: if you ignore them and appear to run away, you will likely appear suspicious and this could then give them reasonable grounds. Although you do not need to engage with them until the point of reasonable suspicion, it is generally not a good idea to completely ignore and run away from a police officer.