A caution is not a criminal conviction, so (assuming that you've not got any convictions for anything else) you've always been entitled to answer 'No' to any question which solely asks about 'convictions' (and doesn't mention cautions).
If a question asks whether you've ever been cautioned in relation to an offence, there's something of a 'legal grey area' about your right to answer 'No'. Because a caution isn't a conviction, it's not covered by the Rehabilitation of Offenders Act. i.e. there's no specific provision,in law, for a caution ever to become 'spent'. However, since convictions which result in non-custodial sentences usually become spent after 5 years, it seems reasonable to assume that a court would rule that you had not acted illegally if you failed to declare a caution after the same period.
The only time that an employer would probably be within their rights to insist that you declared the caution would be if you were applying for a post for which convictions never become spent. These are principally working in, or alongside the 'caring professions'. (e.g. teacher, school caretaker, nurse, hospital porter, care worker, etc).
Chris