£100k in ISA. Keep it there or put into a SIPP

As the title says, I have £100k in a S&S ISA.
My pension looks weak (about £115k) so thinking of topping it up. Therefore, I'm considering opening a SIPP and moving £50k into that.

Ideally, I want to retire at 60. Current situation is as follows:

45m

£46k per annum

Very low mortgage (£80k over 15 years)

As I'm only a basic rate tax payer, I'll only get 20% so not sure whether it's worth it or not as the pension will then be taxed.