My vat registration date is 28th May 2021 (I was late registering) how do I set this specific date as the start of my vat?

No problem at all there, so to enter in a 100% vat transaction you need to do this 

VAT Only/ 100% VAT transactions 

Recording a 100% VAT expense can be a bit tricky in Quickbooks Online, but dont worry, we have a step-by-step guide on how to record it. 

1. click on the + new sign and choose expense 

2. either choose a payee or leave that box empty.

3. on the first line of the expense choose any of your expense accounts. 

4. for the amount enter a pound (1) and choose 20%S as the Vat code 

5. on the second line choose the same expense account

6. for the amount, enter minsu a pound (-1) and choose EXEPMT as the VAT code

7. The total of the expense will show as 0.

8. under the subtotal the VAT box will become editable. 

9. enter the desired VAT amount into the box and save the expense. 

this will create a 100% VAT transaction that will show in ox 4 on the VAT return.