It depends upon the extent of the criminal damage (and also on factors like pre-meditation).
If you simply put your fist through the glass in a bus shelter, after a drunken row with your girlfriend, the chances of the court sending you to prison for the criminal damage, or of activating the suspended sentence, are probably just about zero.
However, if you went down to your old school, armed with several cans of petrol, and burnt the entire place down (causing millions of pounds of damage) you could expect to receive a heft custodial sentence for the criminal damage (which now includes arson), with the likelihood of having the suspended sentence activated as well.
Since you were hoping for a caution, it seems likely that your circumstances will be closer to my first example than my second, so the chances of you being 'sent down' are probably very slim. However it's not surprising that you weren't just cautioned. Most police forces only issue cautions to first-time offenders (or possibly to people who've not been in trouble for many years). If you commit any offences in future, you should expect to be charged rather than cautioned, irrespective of how minor the offence might be.
Chris