I will assume when you mention the Installer, you mean the LiveCD or LiveUSB where you have the Ubuntu installer.
First, if the installer is not giving you the option for F6 it could either be that it booted too fast or that it is damaged (The CD or the Image somehow got damaged).
Secondly, since you do not care about the files or the need to do a backup then I can simply say to boot the computer from within a working LiveCD or LiveUSB, and tell the Ubuntu installer, where it asks what option do you want to choose (Between leaving the current OS intact, removing all partitions and installing it from scratch or doing it manually), to just remove everything.
Already tried right now. My scenario was a system with Ubuntu 12.04 32 Bit. I then proceeded to execute a LiveCD of Ubuntu Server 12.10 Server 64 Bit. It ran, when asked what to do with the other OS I simply selected, remove it and install 12.10 Server alone.
After that I had myself an Ubuntu Server 64 Bit system.
Now, in regards to the 3rd party tools. If this means LiveCD, LiveUSB, Network install and other common methods then we have a problem since, without this, there is no way to install Ubuntu.
Am also assuming this is not a WUBI installation. This would cause additional problems with what I just wrote.