I know I'll be corrected if I'm wrong, but in general, a commoner with "inherited money" is going to rank socially higher than a commoner who has to work for his living.
So, suppose someone is a merchant, and is a very successful merchant, and he becomes very wealthy. In America, he would be looked up to as a self-made man, and respected for his hard work and success, but in mid-19th-c Britain, he would be looked down upon as a parvenu. However, suppose he has children, and he leaves the money to those children. His children would be of a higher social standing than he was-- because they inherited their fortune, rather than working for their fortune. And by the time you got to his grandchildren and great-grandchildren, a lot of people would have forgotten the humble roots of the family's fortunes.
Money would usually be invested in government bonds at a particular percentage. So you have your sum, and it generates interest, and that's the "allowance" that you have to live off.
British government debt was the only security traded on the Stock Exchange at its foundation in 1801, and remained so until 1822.
Going back a bit further in time, just to look at how they got there--
The key piece of skill was the mechanism of “funding” that secured interest payments. When William and Mary re-established the Protestant monarchy in 1688, Parliament wanted both to shore up the royal finances and establish its control over them. Funded borrowing did both. Parliament passed taxes sufficient to pay interest on a certain amount of debt—just over £1m in the first instance—then tacked on a bit more tax to pay down the principal gradually. The monarch could borrow against this “funded” amount fairly easily, but found it hard to borrow more, which gave British debt unparalleled stability. In 1715-70 France reneged on the terms of its debt five times; Britain never missed an interest payment.
and
In 1751 Henry Pelham's Whig government pulled together the lessons learnt on bonds to create the security of the century: the 3% consol. This took its name from the fact that it paid 3% on a £100 par value and consolidated the terms of a variety of previous issues. The consols had no maturity; in theory they would keep paying £3 a year forever.
But then--
By the end of the Napoleonic wars in 1815, the debts of the British government reached £745m. But instead of calamity and national bankruptcy, the rise in debt simply brought a hunger for more investments. As the government itself stopped issuing new bonds, and started retiring old ones through the sinking fund, investors turned first to foreign-government securities in the 1820s and then to shares in the new, capital-intensive companies, such as railroads, leading the industrialisation of the 19th century. Both markets were highly speculative, to say the least. Foreign-government debt included bonds from one entirely fictitious country, Poyais, and most of the new industrial ventures failed.
One of the recurring themes in, say, 1880's-1930's fiction you'll see is how a wealthy family will become impoverished due to unwise failed speculation. Because slightly before your time period-- during the character's father's/grandfather's time, like in Regency fiction-- government bonds would have been a safe and secure and reliable investment, but by the time you get to the 1850's, there's a wide variety of options, but many of them are unprofitable, if not outright scams.
There's a big difference between the nobility (the titled aristocracy, ranking below royalty, but above commoners) and the Upper Class, because there's the money bucket and the social-class bucket, and the two buckets aren't necessarily connected. But it's not until 1894 that the upper classes got hit hard with death duties--- having to pay inheritance taxes--- on top of the usual difficulties of keeping a fortune intact.
So, at the first post, you were talking about nobility/upper class, and at the last post, you were talking about the upper/middle class. Those are three very distinct things, and if circumstances compel someone from a high status to take on a profession or a job that's beneath their station, they generally try to be discreet about it and work under a pseudonym so as not to dishonor their real name.
But if they're people of means, they are less likely to work as we think of the 9-5--- they're more likely to exist as "a private gentleman."
What you might do is find an old copy of one of the Peerage books, and flip through it for fun. See what kinds of occupations tend to be listed, and which family members tend to follow them. It's been a long time since I've looked at one, but I think in general, only the status-enhancing occupations will be listed. So you can be confident knowing that those are "safe" occupations for a gentleman to engage in.
For example, looking at a random page available for free online from Burke's--- it shows that Stephen Abbott b. 1954 is a director and a producer and owns film companies. So to our eyes, that confers status--- unlike being an airplane mechanic or teaching third grade.
If you have an Ancestry account, you can look directly at a database of "
Daughters of Peers Married to Commoners." I don't know how far back it goes, but you can get an idea of what professions/occupations were "safe" or "respectable" for a well-bred young lady to marry into, especially if he was able to provide her with a lifestyle to which she wished to become accustomed.
Or you can flip through "
Kelly's Handbook to the Titles, Landed, & Official Classes" (1875) for a list of what "The Upper Ten Thousand" look like, and see how some of them passed their time. (Judge of the supreme court at Calcutta. Commissioner of the police of the city of London. Honorary physician to the Queen. Deputy-master of the mint. Governor of Bombay. Register of the Order of St. Michael and St. George. Assistant under-secretary for war. Finance minister of Canada. Tutor to the Duke of Edingburgh. And so on.) And then you can figure out which of those career paths are hereditary, or can at least give his son a boost up in life. For example, while someone couldn't guarantee his son would be Physician to the Queen, he could encourage his son to be a fashionable doctor. Someone couldn't guarantee his son would follow him as Secretary to the Railway Department, but could encourage him to be an engineer and get a position as an official. Someone couldn't guarantee his son would follow him as Vice-Consul in the Delta of the Danube, Turkey, but he's certainly got connections to get him a good situation in the Foreign Office.