Why did they reprint them? Why didn't Pocket reprint their own stuff?
And why bother changing the numbers?
The US Bantam novels were published by Corgi in the UK. The US Ballantine novelizations of TAS were also published by Corgi in the UK.
When Pocket started its US publishing program, importing of US books into the UK was quite rare. The novelization of TMP was licensed to Futura. Then "The Entropy Effect" went to Orbit. Then there was a lull for a while. ST II went to Futura. Some of the next novelizations went to Granada (ST III) and Grafton (ST IV). And so on.
Finally Titan bought the UK license. They started by reprinting the Corgi/Bantam novels as "Star Trek Adventures", but in a different order.
http://memory-alpha.org/en/wiki/Star_Trek_Adventures
After the success of ST IV, there was a huge marketing push, even in the US, with "Chain of Attack" (note Pocket US's sudden prominent use of the TV logo in huge letters, compared to earlier ST novels; possibly inspired by the success of Titan's reprints of the UK Bantams). "Chain of Attack" became Titan's #1. Because Pocket US was doing bimonthly ST releases, but Titan UK wanted
monthly releases, Titan started alternating a new, almost-simultaneous release title with an older US title. The UK numbering became totally scrambled. #1, 7 or 17 were novelizations that had previously been published elsewhere locally, so Titan was probably hesitant about starting at that point (although they did end up re-releasing them). In the UK, "Uhura's Song" was a fourth TOS "giant" paperback, to cash in on the great performance of the first three imported US "giant" novels to the UK! That was followed by three more "giants".
http://memory-alpha.org/en/wiki/Titan_Books