It's worth thinking about which of these services you need:
- Book-keeping: Keeping track of paperwork, and movements in cash/bank. Ideally these are entered into accounting software monthly (or more frequently). Some small, cash-based businesses just put paperwork (receipts, and copies of issued invoices) in a shoe box, and give that box to their accountant once per quarter or once per year.
- Statutory accounts/tax filings: If you were a Limited company in the UK, these might include (i) annual accounts filed with Companies House, (ii) annual Company Tax Return filed with HMRC, (iii) quarterly VAT return filed with HMRC, (iv) PAYE (payroll tax) filings with HMRC.
- Monthly management accounts (for you and your investors)
- Planning and advice: at a minimum this would include planning to minimise tax (you can't leave it until compliance time) but may also include cash flow planning or broader business planning
There's more detail around each of these things. I'd be happy to go into more detail about any of these as it relates to UK-based companies.
(I am a qualified accountant but the above is general information and not advice.)