Critical Illness cover

Salam’s

My employer provides a benefit we can opt into and pay from our salaries.

This includes Critical Illness cover - which pays out a cash sum if we are diagnosed with a critical illness. The cover would be provided by Legal & General

Is it ok to opt into this benefit?

Wasalam

May Allah bless you.
If its paid from your salary, then it is not permissible, unless such cover is a legal requirement.

And Allah knows best!

We receive a salary post which we can buy these items. Therefore under this circumstance I would be paying for this cover. Does this make it impermissible?

Yes if you are paying for the cover yourself then its impermissible.
If on the other hand it was a gift from your employer then it would have been different.