Can I enter my past customers data for tax year 18/19 and use this to close this year with HMRC or it works for current or future customers only?

Yes, you can enter transactions that are beyond the starting date of using QuickBooks, szparaga-dominik.

You just need to make sure that the transaction dates are correct. Here's how:

1. Click the Create menu (+) and select the transaction you want to record.
2. Enter the transaction details.
3. Click Save and close.

After recording the transactions, you should be able to close the books. You can refer to this article for more information: How to Close the Books.

I'll be right here if you need more assistance with this.