I have no experience/knowledge of police interviews conducted 'under caution' to know what is permissible and what isn't. It strikes me as somewhat unfair if the interviewee is expected to tell the whole truth and nothing especially when his/her evidence (contents of interview) can be used in court against him/her if it is permissible for the interviewer (police) to knowingly mislead?
In the US lying to a suspect is not a problem at all. If the suspect is telling the truth and has nothing to hide then a lie will not hurt him/her.
For instance, if police lie saying that a friend blamed the suspect a completely innocent suspect would say the friend is lying and he has no idea why he friend is doing it. The lie causes no harm if someone is truly innocent.
An accomplice though might respond by saying the friend is the one who had the gun and that he just drove the getaway car. It tricks him into admitting his role in addition to giving up his friend.
Why is it unfair to trick someone who is actually guilty? American courts have no problem with investigators tricking suspects in this regard.
In Britain it depends on the judge. There is no hard and fast rule of whether this is permissible it depends on the facts of the case. Judges will sometimes excluse confessions obtained as a result of lies.
In the Bamber case the police did not manage trick Bamber into confessing so there was no confession for him to make a motion to suppress. The police lies just help illustrate how he kept trying to make up new things and excuses which certainly doesn't make him look good but he didn't confess so there was nothing to suppress.
There are 3 different bases that judges use to exclude confessions:
1) obtained through oppression
2) the confession was unreliable
3) obtained unfairly
A confession obtained through trickery doesn't meet the definition of oppression but some judges will ignore such and use this ground anyway to exclude a confession
Normally a confession obtained through trickery is in fact reliable. Instances where it is not reliable are cases where a lie is related to a plea deal. In some such cases guilty parties will confess to avoid taking a chance at more jailtime if wrongly convicted. A lie like the one I gave an example of will not be unreliable in the objective sense. A judge might decide to pretend it is unreliable though and use this as a basis to suppress.
The most common prong to use to suppress a confession is the basis it was unfairly obtained. This again is a very fact specific inquiry. There is no hard and fast rule that a tricked confession is unfair. It is subjective and hinges much on the philosophy of the judge handing the matter.
The US is much better because judges don't have discretion to suppress confessions just because they don't like that a defendant was tricked. In turn suspects know police can lie and thus anything they say is at their own risk so they should think carefully and try to be honest if they are going to say anything at all. Normally we advise clients to not say a word though. We want to be present during any questioning of clients and want to tell them what question to answer or not and usually in advance will ask them what they would say to questions so we know what questions to let them answer. We are not supposed to help them make up things to say though we just try to figure out what they would say and try to help them say it in the least damaging way. Ethically we can't knowingly help them to lie, not that some lawyers don't violate such ethical canons surely some do.
The bottom line is that in the UK you don't know how far cops can get away with lying to those they question. It ultimately depends on whether the suspect confesses, if the suspect decides to go to trial maintaining his innocence anyway and if a judge can be conviced the confession should be tossed.