If the prospective employer asks if you have any convictions, then you have to declare it if the conviction was within the last 5 years. As per Rehabilitation of Offenders Act job applicants doesn't need legally to declare any convictions which are spent (after 5 years).The conviction won't get reported in a Basic DBS check after 5 years.
But for specific jobs (working within regulated, vulnerable people (teachers, nurses...etc) will need to have Standard or Enhanced checks which will reveal spent convictions as well. please consider this on the type of job you are applying for.
As it is just a basic check then, as has been said before it is disclosable for 5 years.
It is not an offence to fail to disclose a conviction, but you could well find that the job offer is withdrawn if you have lied on the application. It is not about the offence, but your integrity.