Slots and LHR? Don't get me started! The key to not getting a banning letter? Don't argue with them. Plead complete innocence and apologise profusely and you'll usually be OK.
I can't help thinking that there might be more to your story than you're letting on or have heard. They don't ban companies willynilly, and generally warn operators before it gets serious.
We operate business jets into and out of there all the time and are constantly constrained by the slot system (we were one of the companies that took the slot people to the European Court in 1999), but accept it as a necessary evil. If you constantly abuse it then you should get banned, but I really doubt that you'd get a ban for one 'mistake' - especially as you're a chopper and operating around the landing fixed wing traffic.
We all know that the world won't collapse if helis don't operate to slot, but then if the computer forecasting says it will, you can't really argue it with the person who decided on the limits.
I suspect they shouted at the Co-ordination Manager myself.....