Under S34(1) ITTOIA 2005 expenditure is disallowed if it is not incurred wholly and exclusively for the purposes of the business in question. However, the existence of some non-business benefit arising out of expenditure does not cause it to be disallowed if, in fact, the expenditure is incurred exclusively for business purposes. So, expenditure on the training and development of staff whose relationship with their employer is limited to the employment itself is allowable.
This remains the case where the expenditure is on the development of an employee's skills and attributes which may not be directly related to his or her current job with the employer. Where, on the other hand, an employee on whom the expenditure is incurred has a significant proprietary stake in the business, or is a relative of those who do, there is obviously a much greater chance that expenditure may have been incurred not, or not wholly, for business purposes, but to provide the employee with some personal benefit. If that is the case, then the expenditure is not deductible – the business purpose has to be the exclusive purpose.
To take an extreme example, there could be no allowance for the educational costs of the business proprietor's son who is employed in the business during university holidays. In such cases, you need to assess whether the expenditure would have been incurred on an otherwise unconnected employee doing the same job.