But you realise that much of this is "virtual" wealth, right? It represents claims on economic resources, not £1 billion worth of actual goods and services sitting in a vault somewhere.
A billionaire's yacht is worth £50 million because somebody else (another billionaire) is willing and able to pay £50 million for it. Its underlying real economic content is the labour, materials and productive capacity used to build and maintain it. A mansion might have a market valuation of £100 million, but confiscate it and use it to house the proverbial widows and orphans and it does not magically provide £100 million worth of housing services. It provides however much housing that particular building can physically provide.
The same problem applies more broadly to wealth confiscation. Seizing £1 billion of assets gives the state £1 billion of nominal claims on the economy, assuming those assets can actually be sold at their previous valuations. It does not conjure £1 billion of additional real resources into existence.
You cannot spend your way to unlimited bread and circuses if the farmers aren't producing enough bread and there aren't enough circuses built. At that point you're redistributing claims over existing output, not creating new output.
That distinction between nominal wealth and real productive capacity is rather important, and attempts at mass expropriation (eg. Soviets in 1917 and 1930s) have historically provided some fairly brutal demonstrations of it.