I presume these are two unrelated points?

Nothing stops the UK seizing crown lands. In fact it sort of does. This is where the sovereign grant comes from.

Land however is not generally taxed in the UK so in that sense they do pay the same.

Because they effectively pay 85% tax on the income already. Look up the Crown Estate. The UK government taking it over would result in it being sold off at bargain basement prices to fund a voter winning 5 min fix and there'd be nothing to show for it in 50 years.