Two important considerations: 1. they don't generally cover pre-existing conditions, and 2. they don't generally cover chronic conditions.
I have health insurance through work, but it didn't start until April this year and by then I'd already had 6 months+ of waiting for the dear old NHS to get on with things. Then I was told I'd need surgery and I assumed I wouldn't be covered because of 1, but it turned out my employer had paid extra for the "medical history disregarded" option.
And it also turned out that even though Crohns is chronic, the fact the surgery is intended to correct a condition meant it counts as accute and was covered.
So I had the surgery done privately. But it was with the same surgeon, in an NHS hospital with NHS doctors and nurses and NHS drugs, and NHS food :thumbdown:, but I did get a private room. And I don't think it was done any quicker either as my case was urgent enough I was assured it would be "soon" and wouldn't be the nightmare of being put on an 18month NHS waiting list.
But I'm still seeing the GI consultant on the NHS as that won't be covered, and neither would future tests.
If in future I lose the work coverage I probably would continue paying for it myself. Your health is more important than money at the end of the day, but obviously that's up to you and the NHS will generally keep you alive for free. What you're paying for is not having to wait months and months to see the same doctor, and getting a nicer room if you do end up needing to go to hospital. In theory the treatment is the same.
I suggest you phone up and have a long chat with them and be clear on exactly what you're going to get and not get before you pay for anything.