Hi Rubis,
Can't speak for everyone, but my Base Cdr and I have been with BUPA for some 10 years or so and they have always been reliable. We have both had numerous treatments / ops through them, sometimes with more than 1 claim per year; they have never raised our premium as a result - indeed, they operate an NCD scheme. They're not cheap, especially if you have to fund it privately rather than having it through your company, but that said, you get what you pay for.