Whether or not the London flies to Cygnus Alpha direct or by using 'point to point' navigation (which would explain why it was passing a planet) it is unlikely to have weaponry more serious than that needed to destroy an errant asteroid - and the Federation is unlikely to allow civilian ships to go *anywhere* near a space battle - especially when the military (and civilian) authorities are seemingly unaware of any conflicts overlapping into Federation territory? There may be another explanation - the 'battle' was an illusion created by Zen. Consider: The crew of the London are presented with what they recognise as a computer battle. Almost immediately their scanners are knocked out, while the total damage to the London appears to be fairly minimal. From 'Redemption' it appears that the System's ships can be 'that precise.' The London crew assume that the yet-unnamed Liberator has been damaged in the battle, yet it is operational as soon as a potential crew comes on board (although there is some time between supposed damage and first contact, which would allow the repair facilities to activate). So - could the whole battle be means for the Liberator to 'get more crew'? The Liberator could be programmed to do this as part of its self-repair mechanisms ('Damage? Repair it. No crew? Get more.')
As for the defence mechanisms - could Zen be Sopron-based? The Sopron creature in Harvest of Kairos is certainly able to 'confuse computers.' The central brain in 'Ultraworld' could be another such creature: it acquires information, and Zen says 'information must be acquired not given.'
This does not answer the question of 'why' the Liberator happened to encounter the London in particular (apart from 'otherwise no story').
Further to my comment on Ensor and the System. Ensor is probably close to 70: his son was probably born some time after the Tariel cells were developed. If Ensor was System-born the space station was probably being finished during his childhood (assuming the slaves/System population in general have lifespans similar to that of humans). Federation records of the System might refer only to the old system 'constant conflict between the three worlds, and costs of involvement excessive to gains' - it *would* want to acquire the System's space station and DSV production capacities if it became aware hat they existed.) But the Federation does not appear to be aware of Ultraworld, which must appropriate passing spaceships on a fairly regular basis.
One reason why both Ensor and Avon might have left the System as children (apart from their families disagreeing with it for political reasons). The computer skills of both were evident in childhood and their families chose to withdraw them rather than have them turned into components of the central network.
Jacqui