Adobe have emailed me, notifying me about this.
Here is the email:
We are pleased to inform you that one of your files has been sold.
Selling creative content licenses is an economic activity. Income derived from these sales may be subject to U.S. IRS (Internal Revenue Service) Withholding Tax.
It is important that our contributors comply with tax law. We invite you to visit the Tax Center and to review the information listed on that page.
Until you complete this procedure and the appropriate form is validated, your Adobe Stock income will be subject to automatic withholding at the appropriate tax rate. By submitting the applicable form, you may benefit from a reduced or zero withholding rate on your income.
We suggest you complete this procedure as soon as possible.
So, as a UK-based person, what, exactly, am I to do?
I've never done independent work so I don't know where to start.
Also, Adobe seem to assume that I'm US-based, for some reason.
No, Adobe don't assume you are US based, but THEY ARE. By law they MUST charge a tax on EVERYTHING they pay to EVERYONE - unless suitable paperwork has been filed to prove you are not subject to US taxes. Whether or not they do this, you will also have to declare all your income on your self-assessment form, and pay UK taxes. It's easy to pay double taxes.
You will find this with most stock companies - it is a relatively simple form you fill out stating you are UK which allows Adobe (or whoever) to pay 100% of your sales to you. You then declare that to Inland Revenue in the UK (I believe the first £1000 does not affect taxes but I may be wrong)
If you do not fill out the form then Adobe have to hold back what would be tax in the USA (similar to how an employer in the UK holds back PAYE) while you should still be declaring what is paid to the Inland Revenue.