"four bedrooms, two sitting rooms, and a “nice outlook on green.” The rent was £90 for a year."
According to one CPI calculator (https://www.in2013dollars.com) "£90 in 1919 is equivalent in purchasing power to about £4,951.54 today" and "£700 in 1919 is equivalent in purchasing power to about £38,511.97 today".
Google search also reveals that average yearly wage in London in 1920 was £205 and average wage in London in 2021 was £53,700 so £700 income in 1919 was 3.5 times the average which in 2021 would be £187,950 (quite a bit more than "equivalent in purchasing power to about £38,511.97 today").
A quick web search for 4 bedroom apartment (not sure about sitting rooms) in London shows that rent is between £2,000 and £8,000 per month - let's say £5,000 for a nice one like what Christie's sounds like which is £60,000 per year. They paid £90/£700 = 13% of their (3.5 the average) annual income for rent and now the equivalent rent would be £60,000/£187,950 = 32% of the 3.5 the average annual income.
Even just comparing the plain average income, person in 1919 could rent the same apartment and still have £205 - £90 = £115 to live on for the year (which seems possible considering maid lived on £36 per year but in 2021 the average income leaves you £53,700 - £60,000 = £6,300 short on the rent alone.
So ignoring the tvs, computers, holidays, cars, healthcare and all the things that we cannot compare and comparing what we can, it is obvious that just to live in 2021 we need more money that what we are making and so it is easy to see how most people feel that the living standards have gone down.