I'm in the UK. Does the GDPR still apply to me post-Brexit?

Yes. Here's an article about the situation:

https://www.theguardian.com/technology/2021/aug/26/uk-to-overhaul-privacy-rules-in-post-brexit-departure-from-gdpr

The key line is:

"The GDPR data protection rules introduced by the EU in May 2018 are part of UK law even after Brexit, under the Data Protection Act."