Since you are resident in France and not the UK, the Double Tax Treaty gives the benefit of taxing the gain to France.
The gain is based on the purchase price. So, you need to convert the acquisition cost into French francs in 1972 and then convert the francs into euros at the rate of 6.55957:€1, and convert the sterling sale price into euros at the rate on the day of the sale.
Please note that in light of owning the shares for more than eight years, you are entitled to a reduction of the capital gain of 65%.
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