score:9
Some airlines, especially low cost carriers like Ryanair and easyjet, have passenger documentation requirements which far exceed those required by law. Ryanair in particular is especially infamous for this. Your travel agent will have been aware of this and would advise you to carry both your passport and BRP to ensure that you did not have any trouble with airline requirements which exceed the requirements of law.
Further, it may happen that you unexpectedly need to travel outside the UK for some unforeseen reason. If you have your passport with you, (and already have any necessary visas) you will be able to do this without returning home to pick it up.