Is it possible to finish PhD quicker than formally possible?
I don't understand what you want; do you want us to contact your supervisor for you?  We can't find some magic exception in the rules for you.  Either you need to suck it up and accept this won't happen, or you need talk to your supervisor and convince them that this is a good idea.  

Let me just add that this sounds like a terrible idea to me.  UK Ph.D.'s are way too short anyways; rushing through one will just throw you out into the cold hard reality of the job market totally underprepared.  You don't seem to have thought about the "If" in "If I can finish my PhD thesis earlier, and then move to a preferred place for a postdoc, I do not see a reason to delay." or thought about what happens at the end of your postdoc.  If you are really finished with your thesis at the end of 15 months, then it's not as though you'll twiddle your thumbs for the next several months; you'll have an incredibly valuable chance to start some other projects, and get a bit of a head start before heading into your postdoc.  That's time you'll never get back, and which you want to strain your relationship with your advisor in your rush to throw away.