Who should not request Disclosures (and why)?
Unless a role is specifically exempted form the Rehabilitation of Offenders Act (1974) by being listed in the Exceptions Order (1975) an organisation cannot legally ask about "spent" convictions (which would be revealed in Standard or Enhanced Disclosures) and, should they be aware of the convictions unofficially or knew of them before they became spent, organisations cannot take them into account when considering the ex-offender for promotion or development. Doing so could expose the employer to a tribunal.
Secondly, if an employer asks for and receives a Standard or Enhanced Disclosure for a role that is not exempted from the ROA they may find themselves in a "catch 22" situation. If the Disclosure reveals information that the employer is not entitled to know, the prospective or current employee can sue if they are disadvantaged in any way as a result . If the employer ignores the information and a related incident occurs, any victim could sue the employer for placing somebody with known history of such offences in that role.
In short, unless the role is eligible for a Disclosure, it is best not to ask for one.