Does new visa cancel old visa?

[nfoote]

Just a quick question.

Currently have T1G valid until 1st of Sept.
Will probably apply in person for T1G Exension in July.
I understand now the actual new "visa" will be on this Biometrics card which will be sent out to me some time after my in person appointment.

Question is, what if I leave the country for a week in August before I've recieved the biometric card in the post?
When I return my passport will still have the current UK Visa in it with an expiry date of 1st of Sept, so according to that, when returning during August I would have permission to enter the country.

OR does the new visa held on the biometric card that I might not have recieved in the post before I fly out in fact CANCEL my current permission to enter the country?

More explicitly;
T1G expires 1st Sept
T1G:Ext issued (say) 1st Aug
Does that mean for 1st Aug to 1st Sept I have TWO overlapping permissions to enter, either of which I could use?
Or does the ext cancel the original and I have just one permission from 1st Aug.
OR does the ext actually EXTEND the original and I have one permission until the 1st of Sept and then the new permission kicks in for 2nd Sept onwards?