TFL has stolen my bike

They can remove anything that is causing an obstruction. They must otherwise obtain the bike keeper's consent to remove it.

Fill out a report at the local copshop, ask that the CCTV be viewed.

+1 also worth taking a photo for your records of exactly where it was stolen (either by TfL or others)......

Just to be clear though... I assume you are certain this would have been removed by TfL and not Network Rail who (I think) run the station... might be worth just checking with them too... belt n' braces.... and all that.