The Purchasing Power of British Pounds
from 1270 to Present page, in response to typing in 1750 (Initial Year), 100 pounds (Initial Amount) and 2016 (Desired Year), returns:
In 2014, the relative value of £100 0s 0d from 1750 ranges from
£13,920.00 to £1,638,000.00.
and describes various ways of providing that estimate including (amongst others):
The National Archives has a currency converter for the period between 1270 and 2017: Currency Converter
It shows what you could buy at the date in question for the sum involved, as well as an equivalent today, e.g. for £50 in 1850
In 2017, this is worth approximately: £4,009.31
In 1850, you could buy one of the following with £50:
The Bank of England has an inflation calculator that will try to give you an idea of what a sum of money at a date in the past would be worth now in terms of it's purchasing power.
Obviously this sort of conversion is not an exact science, so I think you have to expect that answers are going to span a fairly wide range.
There are a number of ways money might have been sent abroad. But in your case it would have probably have been via a company.
Around the time that countries and empires had colonies in the Americas and Australia, companies such as Western Union started to emerge due to the increased need (or want) to send money back and forth to family members. These ...
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 ...