I talked about this signing HERE, so I'll get straight to the point.
Harrods' Weibo posted more details about the event:
-> MUSE will be sold starting from 10:00 am, one hour before the beginning of the event
-> Only the album sold there will be signed
-> It's limited to only 200 people
-> Every CD sold there will specially come with a hand strap, and only people with that hand strap are allowed in.
Thanks a lot to "A" for the input and to Carrot for the -very fast- translation.