Comparing currency in different time periods?

Question

In the question How to order copies of English wills (post-1858)? I used as examples these two Probate Calendar entries, which are for a husband and wife. At the bottom I added some notes about the value of the estates, which should have been in a separate question.

ELLIOTT Charles Edward George of The Myrtles Slapton Devonshire gentleman died 13 September 1911 Probate Exeter 28 January to Augusta Elliott widow. Effects £631 13s. 11d.

and

ELLIOTT Augusta of The Myrtles Slapton near Kingsbridge Devonshire widow died 17 April 1947 Probate London 27 September to Caroline Louisa White spinster. Effects £6696 8s. 4d.

Taken as a raw number, it appears that Augusta increased her wealth by an order of magnitude between 1919 and 1947. But the value of estates from these two years can't be compared directly. How can I adjust the figures so they are equivalent in terms of spending power, etc. and to get an idea of the worth in today's money?

The section below the line was copied-and-pasted from the earlier question (asked Jul 14 '14 at 17:58).

For completeness I will also include Verbeia's comment (posted Jul 17 '14 at 10:48):

Devaluation doesn't matter for calculating the current value of amounts of money in the past in the same currency. It would only matter if you were trying to convert pounds to US dollars or some other foreign currency. A better converter for past money amounts into current values for the UK is at the Bank of England.

The previous link is broken so I am assuming it was for the Bank of England's Inflation Calculator.

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Results from the National Archives' Currency Converter:

Charles' estate --

• In 1915, £631 13s 11d would have the same spending worth of 2005's £27,200.82
• In 1920, £631 13s 11d would have the same spending worth of 2005's £13,398.27

Augusta's estate:

• In 1945, £6,696 8s 4d would have the same spending worth of 2005's £173,772.01
• In 1950, £6,696 8s 4d would have the same spending worth of 2005's £152,544.37

(I'm assuming the Archives' calculator takes into effect the devaluation of the pound in 1949, which is discussed in this related question on History.SE, What factors led to the 30% devaluation of pound sterling in 1949?)

Answer 1

With regards to the 1949 devaluation, a book I have about the post-war London stock market refers to a US recession and a substantial reduction in exports to the US.

From The Post-War History of the London Stock Market, by George G Blakey, 1994 edition:

Exports were rising, the balance of payments was moving towards equilibrium with the large dollar deficit offset by US aid, thus taking the pressure off the country's gold and dollar reserves, but the recovery was still fragile and susceptible to external shocks. The shock, when it came, was from the US in the shape of a business recession causing a substantial reduction in purchases of UK goods in the context of an export market where the criterion had changed from one of availability to one of price. There was a renewed decline in the country's gold and dollar reserves and its cost structure was exposed as excessively inflated. Drastic measures were called for and on 18th September 1949 the pound was devalued against the dollar from $4.03 to £2.80.