I just got served a cease and desist from Automattic? What should I do?

First and foremost you should contact a lawyer - HN isn't a good forum for getting legal advice.

If you contact your local bar association they can set you up with a minimally costed consultation ($35 in sf) that will likely answer all of your major questions. This has worked well for me for several issues.

You can try a nominative fair use argument, but it is likely easier to just change the name - are you really getting so much value out of the name it's worth spending thousands on a lawyer fighting it for an uncertain result?

I would like to fight it. I believe everything comes under nominative fair use. Please advice. I am in the UK, they are in the US.