Get down to your local BMW dealer, They do a Motorrad branded version of the ABUS Granit Detecto X Plus 8077 disc lock with Alarm. Great bit of kit and I got mine for less than the standard ABUS version
https://www.abus.com/uk/Mobile-Security/Motorbike-Scooter/Brake-disc-locks/GRANIT-Detecto-X-Plus-8077
I have on of these for sale if your interested? Brand new, and still factory sealed. You can have it for £100.