This answer should only ever be considered on a case by case basis. It is important to recognise whether you or the company would be paying the premiums.
Sometimes organisations offer their employees access to a voluntary health insurance scheme whereby given the employer’s size they have access to lower Health Insurance rates than you could get if you set up a personal Health Insurance plan.
In this scenario you would be paying the premiums for the plan, so it is then simply a case of you working out based on budget and how much you value Private Health Insurance whether you should join the scheme at the group rates provided.
The alternative is where an employer would be paying the premiums for you as part of your employee benefits package.
The company would have set up a policy for employees whether for the whole workforce or a specific group and would be paying the premiums for an eligible employee to be covered by the insurance to have any healthcare treatment they require privately.
It is important to be aware that if you are eligible to join a group scheme and it is company paid, private medical insurance is classed as a ‘Benefit in Kind’ and thus you may well be liable to a P11D tax on the benefit.