Is Financial Services Protection limit relevant for Pensions?

Correct - the FSCS deposit limit protection is generally not relevant for pensions, except if any of the pension is held in cash with a bank (which is not typically the case and your pension provider would tell you about that).

So my understanding is that even if the pension provider went bankrupt, that wouldn't have any affect on the holdings managed by that company?

This is also correct, with the proviso that if a pension provider does go bankrupt, someone has to take over and sort out all their records to figure out who does own what; and there may be some theoretical risk that this might have to be funded by charging the owners of the assets for the work. I'm not 100% certain on the law here and whether it's ever happened before though.