hi,
whether the full caution is administered at the scene depends on many cricumstances. for example if the detained person is struggling, violent or posing a danger to others around him it maybe very difficult for the arresting officer (or their colleagues) to administer the caution. there is case law to suggest that simply taking hold of someone is enough to convey that they are being detained or using words which convey the nature of the caution are enough. the law even allows for no caution to be given AT THE TIME if it is not reasonably practicable to do so eg- the suspect is intoxicated and incapable of understanding, the person is violent, in need of immediate medical treatment, or there is a substantial language barrier.
the whole idea behind a caution to is allow the arrested person (or person suspected of an offence) an opportunity to put a defence forward and to allow the police to potentially use what is said and done after the caution by the suspect/person in future court proceedings.
if we take the example in the first paragraph - the violent person not cautioned at the time because of their behaviour. the evidence obtained from them can still be used because :
- they can be cautioned once the have calmed down
- they ALWAYS have the caution repeated/read out to them at the start of a suspect interview (and prior to charge). this is a Police and Criminal Evidence Act (PACE) requirement. since the interviews are taped using tamper-proof tapes and audio / visual equipment there is no getting away from this. also as part of the formal introduction the caution is explained and the suspect is asked if they understand its meaning. any admissions/actions etc made by the suspect after caution are then put to the suspect for their agreement or for them to deny.