How is it simple?
How does taxing unrealized capital gains work in reality in your scenario? Some guy buys £10 million worth of shares. In 1 year, for whatever reason, it 2xs and he has a tax burden of say, £4 million. He has to then liquidate some of his original position to pay that tax back, everyone has to do the same and all of a sudden there's a dip in the capital markets because everyone is liquidating to pay tax. All of a sudden the price is back down to £10 million.
"Simple" unrealized capital gains simply does not work. There has been modelling on this, the US IRS even looked into this and realized that this is only feasible if rather than the asset itself being taxed, it's all loans and derivatives that use the asset as collateral to be taxed. That way, it becomes a "high value loans tax". This then runs into problems of differentiating between the purpose of the underlying securitization. Are you going to tax someone who using their home as collateral for getting a mortgage/loan to buy another home? etc etc. There are people who are far bigger experts than you and I who have looked at this problem and found it to be extremely challenging.
You mouthing off saying that it's "not that difficult" doesn't change that reality. I've yet to see an actual solution proposed other than, "Yeah well just tax the super wealthy's assets" without it causing problems in all other corners of the financial system as well as hitting the middle class.
In your "not that difficult model", how would this tax unrealized capital gains tax work?