To say I don't have those things?

[WhyCantIuseTheNameIWant]

The nice people at DWP have sent me a letter.
They say they can now inform me of stuff via text or email.
I don't want them to have my email! They can use a real letter.
I don't use a mobile.
Should I send it back, filled in with name and NI number, saying I don't use them?
I hardly check my emails, unless I am expecting something. So post is really the quickest way to contact me.

Just ignore it. If they don't have the details, they can't and won't use them. There are several people in the UK in your position. If they decide to phase out mail eventually, you can provide your email address then.