Consider firstly, if you opt out now, can you ever opt back in?
Will the £100 be before tax or after tax - could be only £80 or £60 that you are really missing.
But, speaking as someone who has, in order, used the NHS, paid for private, claimed on insurance for private and lastly, gone NHS twice - I would say take it.
That's not to say that the NHS are poor, they're not - for the things they are good at, they are very good. But there are plenty of things for which they are only adequate, not medically necessarily, but the level of care and service and comfort.
I'd say take the insurance all day long. It's only a bit more than people pay for Sky + a TV licence or a mobile phone.
Adrian