You were right ersourabhbhola about biometrics invitation. I received it on 15th Jan almost 15 days after receiving acknowledgement letter.

I'm stlll worried, my visa will expire on the 17th january and I haven't got the invitation for biometrics. Its fine, no need to worry. I have applied after 14 Dec and haven't heard anything from HO regarding Biometric details. I think that we will receive a invitation letter for booking appointmen...

Posted application: 20 Dec 2010
Received by HO: 21 Dec 2010
paid by postal order
Acknowledgement Letter received: 30 Dec (dated: 23 Dec)
Biometrics Invitation: Jan 15 (provided: Jan 17)
BRP card and passport received on 31 Jan