Goodness, that's just terrible. Bad enough the stuff has gone, but a "friend"???
Now, plan of action;
Do you know where she is currently staying? Have you any mutual friends that you can find out from? What about the uncle? At this point, unless she's been blabbing, which I doubt, no one should be aware of why you need to contact her so urgently. If you can find her, and I'd say it was more important to find her at where ever she is now staying, as if you just meet up with her in the street, there won't be any chance of your recovering your stuff. You're going to need help with this one, I wouldn't attempt to tackle her on my own. Just remember, if she has the lack of morals to steal from her friend, I wouldn't be guessing that she'll be overcome with remorse and want to just hand your things back...
You need to start making an inventory of all your stuff. Things like your Grandmothers necklace, you you may be able to prove belonged to you? (will/family photo's etc) Would you have any receipts for stuff like your laptop, bags etc. Type it up, making reference to any proof of ownership. This will look more professional to the police.
Once you have your inventory, take your boyfriend with you down to the the police station and say you "wish to report a crime". They are obliged to take all your details, details of the crime and they HAVE to give you a crime number. At any point, if you feel you are not being taken seriously, ask to speak to the commanding officer. This will be taken seriously. As someone already said, because you have a flat mate, it doesn't give them the right to steal your property.
I really, really hope you get your things returned.
I can't imagine how upsetting this must be.
Take care,
K