I am trying to calculate what my investment pot will be in 25 years, adjusting for inflation.
You should really be looking at estimated future returns, not historical returns.
The stock return will consist of three components:
Inflation. Stocks are inherently inflation-protected, so whatever inflation there will be will be part of your return.
The S&P 500 does not include dividends. You should check the S&P 500 Total Return Index.
The average annual total return of the index, including dividends, since inception in 1926 has been 9.8%
This would give you roughly 5.2% annual real return, looking much better.
However, this is in USD. Apparently, one GPB was worth ...