GUP only supports definition updates and not product updates.
Once you assign the install package to a group it will send a small upgrade file ( very small compared to the size of the actual install package ) to all the computers at once that will choke up the bandwidth.
So now you've actually got two workarounds :
1. Create a test group assign the package to that group.
Move 10 or 5 clients ( whatever the bandwidth permits ) to the test group.
Once they have upgraded you can move them back to their original group and bring in the next 10.
2.Export a client install package and copy it wil the clientremote.exe ( which is in CD2 of the downloads) to any of the local servers in london and do a deployment from there.
Clientremote.exe is similar to Migration and deployment wizard.