https://ec.europa.eu/commission/sites/beta-political/files/n...
"Where provided under applicable law (such as within the European Union), you may have the right to ask us to delete Personal Information which you have provided to us [... ] contact our Data Protection Officer at: dpo@userstyles.org."
https://addons.mozilla.org/en-US/firefox/addon/stylish/priva...
Also, the transition period will bind the UK to most EU laws for a few more years.
* http://legislation.gov.uk/ukpga/2018/12/contents/enacted/dat...
The GDPR is UK law automatically because it is a Regulation, not a Directive (which needs to be transposed into national law).
The Data Protection Act 2018 implements the Law Enforcement Directive (as the GDPR excludes that from its scope) and a couple of minor derogations (such as changing the age of consent for children to use websites by themselves to 14).