When you start a new business, you are entitled to claim any expenses which relate to the business within 7 years prior to the start of trading. However, for the amount to be allowable against your profits you need to consider whether the amounts are in fact classed as “revenue” expenditure.
HMRC state that any expenditure that provides new expertise or knowledge to the proprietor should be disallowed for tax purposes, especially when it brings into existence a recognised qualification. This would seem to indicate the expense you incurred on your training course is defined as “capital” and will be allowable when the business is sold, rather than against profits as pre-trading expenditure.
Should you attend more training courses to update your current expertise this can, for the self-employed, usually be regarded as “revenue” expenditure and is therefore deductible provided it is incurred wholly and exclusively for the purpose of the trade.