“That £7,000 is about $100,000 in 2023 money. The 55% bracket kicked in at about $140,000 in 2023 money.”
I believe that UK tax rates in 1978 were even more confiscatory than what you describe.
I plugged in £7,000 in 1978 into a CPI inflation calculator and got the following:
“Value of £7,000 from 1978 to 2023
£7,000 in 1978 is equivalent in purchasing power to about £51,381.33 today, an increase of £44,381.33 over 45 years. The pound had an average inflation rate of 4.53% per year between 1978 and today, producing a cumulative price increase of 634.02%.
This means that today’s prices are 7.34 times as high as average prices since 1978, according to the Office for National Statistics composite price index. A pound today only buys 13.624% of what it could buy back then.”
https://www.in2013dollars.com/uk/inflation/1978?amount=7000
Under the current exchange rate, £51,381.33 is equal to $63,669.53. That means that the 41978 UK 40% tax bracket would start at the equivalent of around $64,000 (not $100,000) today, and the 55% bracket would kick in at the equivalent of a little less than $91,000 (not $140,000) today. And the 75% bracket would start at less than $160,000!
What was the Pound worth in Dollars in 1978? Get that number, THEN apply the intervening 45 years of inflation, that’s how you get an accurate read on what 7,000 Quid then was worth in Dollars today.
Per https://www.macrotrends.net/2549/pound-dollar-exchange-rate-historical-chart the average closing price of the Pound was $1.92 in 1978. That’s $13,440 in 1978. Multiply that by the inflation factor of 7.34, and you get $98,112. Not $100K in today’s dollars, but pretty close.