Firstly congratulations!
The opening balance allocation is a tricky one if you've come from Excel. In answer to your question, yes you can allocate the whole lot to sales or possible retained earnings would be better. The question is are all those funds actually retained earning, if you're VAT registered for example some of that cash held would have likely been a liability to HMRC.
Are you an LTD or a sole trader? If you're a sole trader it's less of a problem where you put the balance, as your tax calculations are all derived from the profit and loss and keeping an accurate balance sheet as a sole trader is challenging anyway given there's no line between you and your business.