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An ISA or SIPP is a tax wrapper, a legal container that determines how gains, income and withdrawals are taxed. It is not itself an investment. What you hold inside it, whether cash, funds, shares or bonds, sets the risk and return, so choosing a wrapper and choosing investments are two separate decisions.
Platforms charge either a percentage of assets held or a flat fee, and which is cheaper depends on how much you hold. A percentage fee is cheaper for small pots and grows with the portfolio, while a flat fee becomes cheaper above a break-even value. Investors should work out that break-even point and review it as their pot grows.
The true cost of self-directed investing is the sum of stacked charges: the platform or custody fee, the fund's ongoing charge, dealing commissions and any exit or transfer fees. Comparing providers on a single headline fee misses the total drag on returns. Small percentage differences compound into large sums over decades.