It depends if you want to be tied down to a 2 year contract. You can buy the phone outright for £340 click. Then find your own sim only deal that will be cheaper than £648. Unless you want to pay monthly because it suits your needs then contract is the way to go.
It is decent.. But still prefer buying s20fe with sim only.. Since your price of contract will go up rpi+3.9% it will impact on the entire price.. Buying s20fe for £400 + £10pm 100gb sim only will cost less than that..