Half of England Owned by 1% of the Population

WHOA!  

"Buy Land, They Ain't Making Any More Of It"
~ Will Rogers

All the land actually belongs to the Crown and that's been the situation since 1066.  You never own your land outright, you hold it as freehold tenancy, it is alienable, that is is you can sell it or pass it on to another on your death but if you died intestate without heirs the land reverts to the Crown.  

Duke William of Normandy granted tenancies to his loyal followers and generally the large land ownership can be traced to the Crown granting lands to loyal supporters over the next 600 years.  20th Century "death duties" reduced aristocratic land ownership to be replaced by institutional investors.

Is that 50% absolute (read: ALL the land) or relative (read: only the land not controlled by the government in some way, whether MOD, National Parks, or National Trust, etc)?

I'd be willing to bet it's not remotely all "Lord Frobbishingly-Smythe's estate around his country seat" type ownership.  I imagine a very large percentage of that 1%-owns-50% figure is consolidated farming concerns.  Much like in the US, you have a bunch of small farms that were gradually bought up and consolidated into much larger (perhaps still "family") farms because economies of scale began to tell.

Since a good chunk of the UK population has zero wish to live outside the cities, that tends to result in rural dwellings (and land) being eventually available to whoever both wants to buy and can afford to do so.  Nigella and Louis, the types of people who complain about such things, tend not to enjoy their time in the countryside, because they despise the people who dwell in the countryside and refuse to change their ways to suit the wine bar/Guardian-reader set.