How to convert an historic value denoted in foreign currency to a modern value in local currency?
For what purpose?

Money over time acts as inflation rate, wage bundle, production good bundle, %gdp, %gdp/capita. Each represents a different “meaning” of money over time.

The US and UK economies have different consumption bundles, wage structures, skill structures, industrial structures, government policy, market agents and social meanings. 1968UK and 2018UK also vary and 1968UK and 2018UK vary in these domains. 

Your question is unanswerable without the commodity or expense, the volume or market (wholesale, retail, government) and the purpose.

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