@Medee, I agree with most of this. In particular I would agree that the all cap, in the long term, is less risky than a UK specific index such as the FTSE100, because you have a broader global exposure and a better long-term industry exposure.
I also agree that all cash is 100% going to lose you money in real terms in an inflationary environment, which means that even if you have more pounds due to slightly positive interest rates, you will be able to buy less with your bank balance in 10 years compared to today, if you just leave all of your money in the bank.
And, I would share your expectation that in any Defined Contribution plan, the individual probably has some discretion around how the money is invested, and it is highly important to use that discretion in order to get a suitable risk-reward profile depending on age and preferences, and also to try to minimise fees.
Where I slightly differ is on the idea that "risk is only volatility" or that it falls to a negligible level with a longer time horizon. I consider risk to be the probability of permanently losing money, measured as purchasing power (so adjusted for inflation), over the investment horizon until withdrawal of funds. Unfortunately that does not fall to zero or "negligible", no matter what you do. But, I believe that risk is definitely lowered by holding a diversified portfolio that includes a lot of equities, but not only equities. For a long time we held equities and cash because bond interest rates were too low to be attractive.
Now, for a 35 year old with no unusual circumstances, in a retirement-oriented portfolio, I think I would have something between 50 and 75 percent in equities (with that wide range going from very conservative to quite aggressive), and the rest in a mix of cash and bonds with a range of durations on the bonds - more skewed to the shorter duration end.
In a money pool intended for house purchase within 3-4 years, I might use a mix of premium bonds and shorter duration bonds. I would only own any equities if the amount were small enough that I could buy the house without using any of that money that had gone into equities, should the stock market be at a low level at the time of house purchase.