Obviously not bothered about preserving any of teh buildings infrastructure.
The only buildings they care about preserving are aristocrat stately homes (essentially a charity for down at heel country house owners - typically NT takes over, & some of property open to public but family allowed to stay in the house (essentially in perpetuity passes through descendants) and have some say in what happens with house & gardens use.
So, recent buildings (and not made of particularly rugged materials and having no aristo connection) have no chance of NT being bothered about preservation.
NT wildlife preservation opens up a can of worms as plenty of NT land allows fox and stag hunts to pass through (and lots of evidence of instances of fox hunts not following a "trail" * but that full on illegal fox hunting occurs on NT land) - and some NT land supports "game" shooting, lots of low wildlife quality NT grassland heavily grazed by sheep instead of being allowed to develop a more complex scrub then woodland ecosystem (and not even bothering to make into more wildlife friendly wildflower rich meadows which is compatible with sheep, just needs less sheep and a bit of effort).
Though in some cases, when they explicitly manage for wildlife (and so do ban hunts & shoots, don't have inappropriate grazing) they do an OK job of ecosystem management.
* Amazing that all these hunts do "trail" hunting - but you never see a trail being laid, and so often the hunts end up in areas that no sane person would lay a trail anywhere near (e.g. fields that are private land t& have explicitly barred hunt from, areas of rural housing where inhabitants have lots of pets around etc.)