18 months ago I had a very similar situation (which can be read about in detail here)
https://www.cyclechat.net/
Basically involved in a hit an run, the offending vehicle was then blocked in by other cars until such time as I got up, cycled 200m to where the car had been blocked in, and approched the driver, who denied anything was wrong and made to drive off again, I reached in to get the keys, the key broke off in the lock, but I had the rest of the bunch.
I then phoned the police, it being the Friday before Xmas despite waiting around for 45 mins after the accident they cound not get a car there to take charge.
Eventually the despatcher said I had to give back the keys, and go to the cop shop and make an official complaint.
End result is nothing was done about the driver at all, police dropped the case (I had witnesses etc)
So whilst I would say I was on firmer ground than you were in removal and breaking the key, even I had to give them back.
I looked into the cost of barrel replacement (as apparently it's almost impossible to remove the remains of the key) as I was interested to know what the cost would be if they ever chased me for 'damage' (they didn't). Replacement of the barrel of a typical car is over 100 quid! as well as the issue of the car being undrivable until work is completed. So you have caused a fair bit of damage.