But why the doubt?

Sorry to be pedantic but there's no such thing now as G.M.T.  The ISO is UTC (Coordinated Universal Time but it's in French),  The Canaries (Spain) keeps the same time as UK as does Portugal and Madeira.  Fortunately all the clocks change at the same time each spring and autumn.   I used to have to work out local clock flight tickets for charter flights from UK to the Canaries then on the Lanzarote and then to Madrid at the time when Portugal and UK changed their clocks one week adrift from Spain.  There were, I shudder to remember, a number of coockups !!!!  Hooray for stanardization.