First of all, go through either tcb or Quidco to earn £35-£50 cashback on this deal, taking total cost of this phone down to £74 or £59 if you get the £50 (obviously not guaranteed)
Still a great price even if the cashback doesn’t pay out, obviously a bonus if it does.
Cancel the sim within the first 14 days by ringing the network, not cpw, the network, and telling them you want to cease your contract.
If they refuse, then live chat them, if they refuse then ring them back, they have to let you cancel.
These deals have been posted umpteen times in the past, and have proven they work, as @jazzuk777 Or @Harefoot t or @WajAllRound can prove to name but a few.