Think the problem is its an unrealised gain until you sell the shares/bonds and take it out of your pension. In which case you might as well just have a higher tax rate for pension drawdowns over a certain amount. If the pension grows and it isn't taken its clawed back by IHT anyway.
Now if we are looking at unrealised capital gain then there is a much larger prize which affects the seriously wealthy. That being art, cars, wine, whiskey, watches, Class A shares etc which are all bought as investments, held and untaxed, or in some cases (Class A shares) used as leverage to borrow money against at preferential rates using securities based lines of credit or similar lending tools for the wealthy.