British English Decoder— Can You figure it out?

Your British friend says they left their brolly in the boot of the car.
What are they talking about?

Their brother is sitting in the back seat

Their coat is on the passenger seat of the car

Their umbrella is in the trunk of the car

Your British friend says, "I'm really mad about that flat!" What does she mean?

She is furious because her car got a flat tire

She ate some bad flounder at a restaurant 

She is frustrated because her hair is too flat

Your British coworker says he left his jumper on the tube. What does he mean?

You're in London and you see a sign that says: Mind the Gap. What does it mean?

You need to think about shopping at a famous department store

Don't leave too much space between people when standing in line

Be careful stepping between a train and the platform 

Your British friend says they left their brolly in the boot of the car. What are they talking about?