The pound did not change its value in 1971 as a result of decimalisation. Nor did the shilling or the guinea, in fact - only the penny and amounts derived from that.

The reason £25,000 was then worth $125,000 and is now worth less is the fluctuation of currency prices over time. You can see a graph and some macroeconomic reasoning for why the dollar has continually strengthened against the pound at, among other sources, http://www.miketodd.net/encyc/dollhist.htm

No, this was about the exchange rate at the time.

As sterling has devalued over the last century it correlates nicely with the decline of British industry, especially manufacturing. Yet you still quite often hear some business group spokesman (CBI etc) on the news hoping for a devaluation of sterling so they can remain competitive.

100 years and they still haven't figured it out.