Travel without Biometric Residence permit

[help_me_please]

Hello,
Yesterday I recieved an extension for my Tier 1 General Migrant work permit and was informed that a Biometric residence permit card would be posted to my home address int the UK within 7 working days.

I need to travel back to my home country urgently in 2 days and come back in 2 weeks. My old visa has expiry date 28 July 2012 printed on it.

Will it be valid for me to travel to my home country and back with this visa (so that I do not have to wait till I recieve my Biometric card? since it is a bit urgent for me to visit home)..

I am concerned that my new visa might have superseded the old one and may not show up on the systems or immigration office/airlines. Please guide one on this.