Just an update - I still have no news for applicants' interview.
After few emails and phone calls to my local MP (elected), I got the following depressing reply.
As per the CIC website, the published processing time for spousal sponsorship applications in the Visa Office in London is approximately 28 months from the day it is received by London. London is processing an overload of applications from the European countries as well as from Pakistan and the waiting period you are experiencing is not particular to your case but is a general situation in which many applicants are facing. Your application is well within the published processing time and the applications that were received before your spouse’s will be processed first before your spouse’s interview could be scheduled. As such, the immigration officer can only interview a limited number of applicants on each area trip and the priorities will be set based on the date the applications were received.
I understand that long processing time is frustrating to many applicants who are waiting to re-unite with their families. However, please be advised that the situation you are in is not uncommon or irregular.
Thank you very much Mr. no use MP.