Good option for buying a Samsung S8 / S8+ model interest free by spreading the cost and then getting a sim only deal from your trusted network.
through Paypal Credit.
I believe your this will be open to all networks.
example S8 price:
A £689 purchase would be an instalment offer of 24 monthly payments of £28.71, with a total amount payable of £ £689. The cost of credit is £0 and the rate of interest is 0% p.a. (fixed).