You'll have to log in to your QuickBooks Self-Employed (QBSE) account through a web browser, then select the personal trips you want to delete from the Miles page, @gilesyone-gmail-. Let me guide you how.
The Miles page tracks all your mileage information. From there, you can edit, delete, and update the type of trip you have (from business to personal or vice versa). With this, you can filter all your personal trips (300) from the Type drop-down menu and delete them from there. Here's how:
Go to the Miles menu.
Click the Type drop-down arrow, then select Personal.
If all 300 trips are included, select the box beside the Date column to select all of them. If there are other personal trips that you don't want to include, just deselect the box beside it.
Select the Delete button above.
Click Yes to confirm.
I've attached screenshots below for your reference.
Additionally, you need to categorise your trips accordingly. This way, you're able to submit the exact amount for your self-employed taxes. For tips to keep your transactions accurate, I'd recommend checking out this article's Track trips accurately section: Fix mileage tracking issues in QuickBooks Self-Employed. It also includes steps on the things you need to do when you encountered mileage tracking issues.
Please know that you're always welcome to comment below if you have other concerns or follow-up inquiries about managing your trips in QBSE. I'm just around to help. Take care.