Samsung are discounting 20% off the S24 and a further £200 if you send them any phone (doesn't have to be working). There is also a cashback incentive running that will give you £150 in your account which would brings the total cost down to just £289.20. 
Samsung Galaxy S24: £639.20
£200 guaranteed trade-in discount: £439.20 
£150 Samsung cashback: £289.20

To get this deal you must have access to the Samsung EPP site through the organisation you work for or people with a TOTUM student card can also access the discounted Samsung site from via the Totum site.