I might be missing something here, but isn't it just a case of signing-up to HMRC's website for self-assessment taxpayers, fishing out one's records of income and expenses (either held on computer file, if yoyu do that, or on paper) and filling-in the boxes on the screen?
All the little boxes have 'help' icons associated to tell you what data goes in the boxes. At the end of all that, HMRC works out the tax liability.
The alternative is to fork out for an accountant to do the same thing for you, but that seems such a waste of money.
What else can possibly be involved in tax software? - there isn't a programme that can tell you what allowances you can invent to try claiming for.