(2-part post):
Once a conviction is 'spent' you don't have to declare it on most applications for employment (see exceptions below). So, if an application form asks "Have you ever been convicted of any criminal offence" you can legally answer "No" without committing a further offence of fraud. (i.e. the law allows you to lie without threat of retribution).
Some jobs are, however, exempt from the provisions of the Rehabilitation of Offenders Act. These are generally jobs working within, or alongside, the 'caring professions'. So any job working with children will be exempt, as will most jobs within the NHS. The fact that an exemption applies will always be clearly stated on the application form. In this case, you wouldn't be allowed to answer "No" when asked if you've got any criminal convictions. (If you did so, you could be charged with 'seeking to gain a pecuniary advantage by deception').
The terms of the Rehabilitation of Offenders Act apply to things other than employment. With a conviction for fraud, it's extremely unlikely that you'll be able to get anyone to insure your house or its contents for the next few years. (You'll have to declare the conviction on any application form. Failure to do so would invalidate any insurance obtained and might lead to criminal proceedings). Once the conviction is spent, however, you can safely 'lie' and write 'No' on the form.