401(k) and IRA accounts are ways to invest in the market and accumulate wealth for retirement. The amount you’ll have at retirement depends on what you put in and how the market performs (i.e. its cumulative gains and losses). At retirement, you’ll start spending the money you saved, hopefully with the right budgeting so you don’t run out.
A Personal Pension, on the other hand, generates an infinite amount of guaranteed monthly income that lasts as long as you do. Instead of being invested in the market, the Personal Pension is a portfolio of income annuities guaranteed by life insurance companies. It’s a way to prepare for retirement without worrying about the market risk or how long you’ll live.