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Did they ever explain the origins of the Liberator and/or what happened to the previous crew besides Avon's "...live rocket launch control. It has been activated." in Space Fall.

"Redemption", the first episode of series two, is where the crew meet the System and its slaves who are known as the Atlas who refer to the Liberator as "Deep Space Vehicle 2".

As shown in this image, the System persuit ship single "nacelle" design and technology is just like the Liberator.

The System is revealed as the creator of the Liberator (Deep Space Vehicle 2) and another Deep Space Vehicle identical to the Liberator is sent in persuit.

The Liberator was found drifting and Leylan surmised that there had been a battle involving "two fleets, maybe more", but the exact circumstances of the conflict and the fate of the Liberator crew were never revealed.

There is an excellent audio drama called "Escape Velocity" an official Blakes 7 Audio Drama - which starts when the original owners of The Liberator have taken it back (from series 2 episode 1 of original series) and they are reading the computers log. It reveals that before Blake got it the System had brainwashed humans as crew who piloted the ship in their war against an inter dimensional alien race. They were in a battle with them and damaged and the brainwashed crew got memories back and mutinied when the damaged ship lost connection with the system that controls it. The Ships self defence mechanisms ( Zen) killed the crew - all except the pilot who suicided and then Blake found it drifting. Zen was a bit distorted and cut off from the System so it linked with Jenna and perhaps because off the damage Blake was able to be accepted as crew by Zen.

Ok, so a quick scan through Episode 2 "Space Fall" which doesn't revel much other than it's "abandoned". A search on Wikipedia seems to confirm that there was limited backstory. Basically there's nothing much said about the "aliens" that Liberator belonged to.

From Wikipedia

Whilst awaiting deportation from Planet Earth, Blake meets thief Vila Restal and smuggler Jenna Stannis. On board the prison ship London, Blake meets convicted murderer Olag Gan and computer engineer and embezzler Kerr Avon. The London encounters a battle between two alien space fleets and the London's crew plot a course to avoid the combat zone and continue their voyage. They encounter a strange alien craft, board it and attempt to salvage it but are thwarted by the alien ship's defence mechanism. The commander of the London sends the expendable Blake, Avon, and Jenna across to the ship. Blake defeats the defence system when it tries to use memories he recently discovered were false. With Jenna as pilot, the three convicts escape in the alien craft.