Which expenses line for a capital expenses

QBSE does not have a balance sheet which is where you would record the purchase, and later the annual depreciation, of a capital asset.  A capital purchase is not an expense.

So the short answer is you can not record that in QBSE

As a sole trader in the UK you can use traditional accounting or cash basis accounting.  If you are using cash basis accounting, you may be able to offset the cost of machinery as an allowable business expense.  I would recommend checking the government's website advice for clarification : https://www.gov.uk/expenses-if-youre-self-employed

Having reviewed their advice, if you consider it to be an allowable expense but can't find any other category which it neatly fits into, I would suggest using 'Other business expenses'.