Hi @rebeccaleggett-b,
For QuickBooks Self-Employed (QBSE), it will start to calculate your taxes at the start of your self-employment date.
If you have transactions before your self-employment date, then these will be ignored in your Self-Assessment Summary. To check your self-employment date, follow the steps below:
- Go to the Gear icon.
- Select Tax Profile.
Check out the screenshot below for your reference:
As for the changes brought by Brexit, you can view this article: Brexit FAQ. It's a list of common questions our users have for QuickBooks Online and QBSE about this process, along with their answers.
Post a reply if you have other concerns about your tax profile in QBSE. I'll be around to help you out.