It is very important to distinguish between the underlying investment, and the account type in which it is held. Account types include ISA,Pension (for instance SIPP or employer sponsored), and taxable account.
Within a pension, unless the employer / plan sponsor gives no control over how it is invested, you can normally choose what proportion to hold in cash, bonds, stocks, as well as what kind of bonds or stocks. So it should not be necessary to take money out of the pension in order to choose lower risk investments. For a person wanting to hold cash or something quite similar to cash in terms of interest rate risk exposure, it should be possible to buy a short-term bond fund. That way the benefits of the pension wrapper are retained while the underlying investment profile is shifted.
Each account type has pros and cons.
With a taxable account, the funding is with after-tax money and all gains and income are also taxed upon realisation, but there is the widest choice of investments available and it is usually possible to get the lowest fees.
With an ISA, the funding is with after-tax money so there is no tax benefit at the point of investing, but all future income and gains within the ISA are then tax free, and the principal has of course already been taxed. Money can usually be taken out any time it is needed, but it cannot be put back in. There is a limit to how much can go in each year so it needs to be built up over time, and there are usually more limits on choice and higher fees compared to a taxable account.
With a pension, the funding is with pre-tax money so there is an immediate tax benefit upon investing. Income and gains within the pension are tax-free. However, when money is taken out in the future, it gets taxed as ordinary income (except for the 25% TFLS). So capital gains within a pension are taxed more heavily than gains invested in an ISA or even in a taxable account. There are also more limits on choice and higher fees compared to a taxable account.
Despite the limits mention, it should be possible to own any of bonds, stocks or cash within any account type.