You won’t get HALion Sonic 2 for free with Cubase Artist 7.5 free update.
You will get HALion Sonic SE 2 for free with Cubase Artist 7.5 free update.
So if you have HALion Sonic (ver. 1), and want HALion 5, you would need to update from HALion Sonic (ver. 1). I am seeing it as $200 US (probably 182 GBP for you).