The difference between flexi-access drawdown and UFPLS is chiefly that, with a UFPLS, you aren’t withdrawing money from a drawdown fund at retirement. Instead, you take money direct from your pension pot rather than moving it to drawdown first.
With UFPLS, the cash you take from your pension is immediately taxable, with 25% of every withdrawal being tax-free and the remaining 75% being subject to tax at your highest marginal rate.
With drawdown, you move the money to a drawdown fund first before withdrawing it. Anything above your 25% tax-free cash lump sum isn’t taxable until you choose to take it from the protection of your drawdown fund.
At this point, it becomes taxable at your highest marginal rate in the tax year you choose to take it.