I’d definitely move away from Excel now if things are getting messy. For a complete beginner in the UK, you can opt for FreeAgent, QuickBooks, Xero or Sage. Personally, I’d recommend FreeAgent. It’s pretty easy to get your head around and covers invoicing, expenses, tax, VAT, etc.
QuickBooks is another good beginner-friendly option, while Xero is worth considering if you expect the business to grow more. Just make sure whatever you choose supports Making Tax Digital, so you don’t have to switch again later.