BIM42526 - Specific deductions: administration: own training courses
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Provided it is incurred wholly and exclusively for the purposes of the trade carried on by the individual at the time the training is undertaken, expenditure on training courses attended by the proprietor of a business (either as a sole trader, or in partnership with others) with the purpose of up-dating their skills and professional expertise is normally revenue expenditure, which is deductible from the profits of the business. See BIM35660.
Business purpose test
In considering the question of purpose, you should not take an unduly narrow view of whether the content of any particular course only up-dates existing skills of the individual. But if it is clear that, for example, a completely new specialisation or qualification will be acquired as a result of the expenditure, it is unlikely that the expenditure will be wholly and exclusively for the purposes of the existing trade. For general discussion of business purpose see BIM37050 onwards.
Expenditure on new skills etc may also be capital if what is acquired can be viewed as an identifiable asset of sufficient substance and endurance. See Dass v Special Commissioner and others  EWHC2491 (Ch), and BIM35660.