As someone who recently did this on the train and got stopped by british transport police. It helps to know the law.
When requested to open said bag/case make sure you are aware the second you open the case you are now brandishing a weapon in public and can be arrested. You are best off requesting they take you to a secure or local police station before you open the bag/case. Do not at any point open or do anything that may make your firearm visible to the public this may cause undue panic and cause the police/btp to arrest you.
You dont need to tell people about the firearms but don't act like you are trying to hide it, this can be considered a threat. Be calm and inform anyone who asks that its a AIRSOFT GUN and not a real firearm.
Do research ahead of time and know if you are going to enter a transport hub and normally these are in heightened security alert, make yourself look low profile if you cant then make it look obvious you are going to airsoft, e.g. camo on, boots, branded clothing. Most police understand and know what airsoft is..
Take cation what you say on the phone. Dont talk about tactics and training, as people may over hear this.
Hope that helps.