Anything is possible. There are past examples of this. A couple of people inside the company could take fraudulent/illegal actions. We would not know until after the fact. There should be checks and balances, and systems to make it impossible. But insiders can probably workaround them too. I think like others it would be extremely unlikley. But that said, I feel slightly more comfortable having pension in two providers!
Equitable Life promised very high annuity rates and couldn't pay for them when the time came due to a fall in interest rates
With capitalisation requirements at the levels they are at and the constant Central Bank checks going on, it is very unlikely that one of the main life assurance companies going bust. If it happened, it would more likely be with one of the smaller, self directed providers.