I would buy the Allez, ride it as much as possible. See how you get on with the double, see if there are any aspects of the ‘fit’ that don’t quite work. You know that you can sell it again for £400. Once you have a bike and ride it, it will soon become clear if it is the right one.
Then the next time you are near a decathlon try the Triban. As you will be used to the Allez, any differences that are good / bad will become clear. If the Triban is better, sell the Allez and buy the Triban???
Maybe a bit of hassle, but if you can’t decide between the two, the only way to choose is by fit and comfort.