Bit convoluted this deal but worth it Go to a main O2 Store and ask about the P20 Pro (or mate 20 Pro) on 60gb contract.
Say you want to pay phone off outright.
You will get £100 off their current price of 220 for the P20 or 259 for the MATE 20 Pro. Contract is 30 days but can cancel withing 2 days using pac code or phoning up for little or no charge. Depends on operator.