I FIXED IT!!
For the references of future generations, heres how:
The problem was that by plugging the a clone of the system I was running into it, I'd "confused" my system as to which drive to write to, and somehow nuked the Volume Group metadata for the original internal drive. Luckily, I still had the metadata from the clone I'd made.
This meant that I could plug in the working backup, unlock it, and use vgcfgbackup to make a copy of its metadata.
~$ sudo vgcfgbackup -f /tmp/trousers
Volume group "ubuntu" successfully backed up.
This creates a text file with the metadata in /tmp called trousers (called it whatever you like, makes no difference)
Then, I dismounted and unplugged the functioning backup clone, and plugged in the borked internal drive. Unlocked it like I did the backup, then run:
sudo vgcfgrestore -f /tmp/trousers ubuntu
Restored volume group ubuntu
Upon doing this, the volume group suddenly sprang to life in disk utility, and the borked internal was unborked, mountable and readable again!
Critical bit here is that 'ubuntu' was the name of the original volume group. You can verify this by opening up the 'trousers' file in gedit and looking at the entry on the line below "creation_time".
Also worth noting that this only worked because the backup was an exact clone of the original internal, and hence the VG metatdata file had all the correct UUID's in place. I actually got an error message first time round because I'd originally manually changed one of the UUID's on the external drive. I had to use pvdisplay on the original internal to find its UUID, and then manually edit the 'trousers' file to get it to work.
Anyway, hope that helps someone one day.
Best Answer
According to the screenshot, the reason of dropping to initramfs is that root partition was not found. There could be two reasons for that:
To verify if it's #1 or #2, run the following commands in initramfs console:
If the UUID is found then it's #2 (LVM was not loaded by initrd), and you should follow this link: Can't find LVM root dropped back to initramfs
If UUID was not found, you will simply need to provide the correct UUID in root parameters of your GRUB menu.
To figure out what the right UUID is run blkid to see what is assigned to the root partition. There is usually a 'root' word in the partition mount point, e.g. on my machine blkid would show something like this: