If you get the £9 sim only offer for 4GB, Unl min and text you can change it to the £20 8GB offer and retain the £8 discount (£12 month after discount) from the original offer through the mythree page or the Three app. Change plan on the Allowance tab by clicking "Change Price Plan".
This is for those looking for UNLIMITED MIN and not necessarily want the Feel at Home or Tethering. Otherwise the 600 min 8GB for £12 is better option.
Possible £45 TCB but not sure if price change will make any difference.