Suspects can only receive a caution if they ADMIT THE OFFENCE. I've written that in capital letters because it is very important. You must confess and show remorse, or at least an understanding that what you did was wrong, to be eligible for a caution. You cannot receive a caution if you deny the offence in anyway, e.g. by giving an alibi or putting forward a defence.
There's a lot of nonsense talked about inappropriate cautioning, which distracts from the real inappropriate cautions. The typical example you'll hear is that in the past year 23 (or whatever, I'm making a number up here) people were cautioned for rape. Think of rape and you think of a demented man grabbing a woman in a dark underpass, holding a knife to her throat and forcing her to submit to sex against her will. That is rape and I seriously doubt that any offence with those particulars has ever been dealt with by way of a caution.
Cautions for rape are very rare. They are usually given out because the prosecution are not going to be able to prove the allegation in court, maybe because the victim does not want to go to court or maybe for another reason. I suspect more often cautions are given out for "offences" that you might not expect, as in my next anecdote.
A couple of years ago I dealt with a "rape" of a girl under 13. My client was her "attacker" and I know, you're thinking I'm a nasty bastard for using the quotes on those two words. But, this "attacker" was a tiny and very scared 11-year-old boy. He gave an account, which I completely believed, whereby he had been shanghaied into going into a block of flats with this girl where she had performed oral sex upon. He told me he was nervous about what was happening, but nonetheless went along with it. Teachers at school found out, informed the girl's mother and between them the police were called.
After I had left the police station and against my advice he, on the advice of his mother, accepted a caution (for kids they are known as reprimands and final warnings, but it's basically the same thing) for rape - on a separate point I really do believe that at least 50% of parents should never be allowed near their child in custody, but that's a whole other post. Because of his caution he is now a registered sex offender and must notify the police of his address. That is a wholly inappropriate caution for rape, but not for the reasons the press would have you believe.
Last weekend I represented a couple of men who had been arrested for affray. They told police they had been attacked outside a night club by three men. They had serious injuries to prove it. Despite putting forward an account in which they were the victims of an unprovoked assault they both accepted cautions after I had left the police station.
It seems to me that cautions are regularly being used to dispose of cases that where securing a conviction would be very difficult, if not impossible, but where the suspect is vulnerable to the promise of a speedy conclusion. I assume that this boosts detection rates and is much cheaper than prosecuting.