I am planning to purchase a phone for £25 upfront payment and £25.52 and over 24 months it is £637.48 for the iPhone 7, however, the Buy It Now price is £539, I am aware this includes interest but is this considered as riba as I am simply just trying to purchase an item
This is a really great question. As you will see on the following article, the majority of scholars, including the four Imaams, were of the view that it is permissible and they provide their proofs from the Quran and the Sunnah. Some ulema, whoever, rule such transactions are prohibited on the grounds that it is the same as Riba. They argue that the increase in price in return for the deferment is a form of Riba. From what I have read on the topic the stronger opinion, given that it is also the held by the majority, is the former one. You can read more details at the following link https://www.islamweb.net/en/article/135440/paying-through-installments
There is also a discussion of this topic in Majma’ Al-Fiqh Al-Islami which rules such transactions are permissible. I think it is online by google-ing "paying in installments Islam" Allah knows best.