Whether premiums are a P11D / benefit in kind depends on how you set up the shareholder protection policy. In brief:
On a life of another basis: HMRC does not typically consider premiums a P11D benefit
On a company share purchase basis: HMRC does not typically consider premiums a P11D benefit
Own life under business trust: It depends on whether the company pays or the individual pays. HMRC considers Shareholder Protection a Benefit in Kind when the company pays for the shareholders, but not if the shareholder pays themselves.