I think you mean £20 billion for that latter figure. This is largely because a significant amount of assets are held in ISA's (£20k a year contribution per person allowed) , or via personal property which is capital gains exempt or in a pension which is again, capital gains exempt.

Thus only the wealthiest are outside these boundaries, and they often will not liquidate holdings until their death to pay inhertiance tax, or in trusts which will liqudiate over decades as they can pay inheritance tax over a very long period.

This is not to mention the large amounts of off-shore holdings.

Many people opt for off-shore bonds (which have a number of advantages) which means paying normal tax instead of capital gains, so the capital gains figure doesn't really capture investment as a whole.