There is no activating trains or any messing with stupid apps with Britrail passes.

Thanks for the answer . I‘m a little bit confuse , can i activate with my pass on the same day or i have to do this a day before in the Hotel where i have to stay ? Kindest Wolfgang 

What pass do you have?

Hello Anna I have an Interrail Great Britain Pass Kindest Pollux 

Interrail Great Britain Pass is an entirely different thing to Britrail pass so my response earlier is incorrect.

Yes you can and should activate a travel day only on the day itself, you can add the train(s) you plan to take to “my trip” section beforehand and only click the button to add to pass when you are certain you will be taking the train.

Hello the answer was been helpfull have a fine day . 

Hello Anna the rail strikes in netherland to the end of the week . Not possible coming to UK until the 6thof Sept. How i can cancel my Date 30.8. and put it of to 6th of Sept. ? Kindest Wolfgang 

Sony Anna i have change . Have a good day -Wolfgang 

Just cancel your travel day in the app. Do it today before midnight CET! Go to My Pass, select your travel days and then tap on "Cancel travel day”. Do this for all future travel days. If tomorrow would be your start date, then also answer yes to the question whether you want to deactivate your pass.

This is one of the reasons you should not activate your pass or travel days in advance.