R97 and others who are interested recommend a good read; the The Decline and Fall of the British Aristocracy, by David Cannadine.
"At the outset of the 1870s, the British aristocracy could rightly consider themselves the most fortunate people on earth: they held the lion's share of land, wealth, and power in the world's greatest empire. By the end of the 1930s they had lost not only a generation of sons in the First World War, but also much of their prosperity, prestige, and political significance."
Repeal of Corn Laws in 1846 was one of first salvos over the bow if you will; then came WWI, increased taxation (on land, death duties, etc....), and end of fee entail. By 1920's or so the wealthy in GB were paying some of the highest taxes they had ever known. So much so many simply could afford to keep things up and began selling up bits of their estates if not entire family piles (seats). If no one would buy they simply took roofs off those stately homes and let nature and time do demolition for them.
Much of damage caused to various noble families was caused by themselves. Besides often being rather dull, they still often hung onto a class system that believed a "gentleman" or "lady" did not work or engage in any sort of trade. Their income was largely all passive (investments, rents or whatever from land), and often not managed very well.
When the new heir Matthew Crawley arrives at Downton Abbey and begins getting involved with the estate he's shocked at how badly things are being run. Here the current earl is moaning about how he's strapped and might have to sell up, but Matthew discovers waste, inefficiencies, and other issues that were contributing to heavy losses. Attempting to touch subject with his wife and FIL he got same response that would have come from real life aristocrats; "you don't understand our ways, the estate was never meant to be run as a for profit business......".
Fiction is rich of examples as well; when the common and grasping Rex Mottram tells Charles Ryder that the Flyte family is "wealthy as such people are", but that they "are in for a jolt if they don't watch out", CR dismisses it as more of RM's lower class origins and his scheming social climbing.
Of current nobility who are worth millions or billions it is largely because of good investments, but also that happily they "own" some of the most high priced real estate in England. Duke of Westminister owns more land than the Queen, and this includes 300 acres of prime central London land in Mayfair and Belgravia .