Sorry to hear about your news, I can think of two options :
1) if car is due a service they will then note that car has a busted wing mirror, if service isn’t for a long time then you could put off replacement until service.
2) go on eBay, or google car breakers and see if you can pick up a good one in the same colour and fit it yourself, will save you money than having to pay 100 excess, and if it has a couple of minor scratches on it you shouldn’t lose your good condition bonus come the end of the lease.
do you have cctv or do any of your neighbours have cctv that caught the little sod swipe it off?