I doubt this is correct. Your gain (or loss) is the change in value of the share plus the change in the exchange rates. If you paid for your shares in euro, the increase in value of sterling vs the euro will increase your returns in euro terms.
For example, if you bought a share at say 5.68 GBP this time last year and today the share is now worth 6.56 GBP, you have a gain of 15% in GBP terms. But as you paid for the share in euro, last year you paid 6.9889 EUR for the share at the prevailing GBP/EUR exchange rate.
In the past year sterling has increased in value relative to the euro. If you sold today you would get 8.9932 EUR for the share at the current GBP/EUR exchange rate.
As the euro has decreased in value relative to sterling your gain in euro terms is 28%. So you are getting a double gain, one from the increase in the value of the share and one from the increase in value of the currency in which the share is denominated relative to the euro.
No I recently purchased shares in Tullow via TD at a client settlement currency rate of 0.69559, rate when sold was 0.745 therefore even though the share price has risen, the gain has been lost on the exchange rate.