Change My Mind by Sophie Thomas

Adrienne Henry is coming home.

When Addie is offered an English professor role that is too good to refuse, she finds herself packing her life up in Montreal, where she has lived for over a decade, and returning to London.
Eli Jenkins is a burnt-out chef, living a lonely existence. In search of a fresh start, he moves from Manchester to London, a city he never saw himself returning to after he was cut off by his family. But he’s found a job that might help him achieve that elusive work-life balance.

To help ease the transition back to London, Addie’s mum, Vivi, offers Eli and Addie a flat to live in, so long as they’re both fine with a flatmate.
They are more than happy to share the space, but little does Addie know, her new flatmate is the former school rival that she hated passionately for the two years they shared a classroom.

Now adults and living in close quarters, Addie is starting to realise that the line between hate and lust might be thinner than she was willing to admit. As the sexual tension between them increases, they agree to get it out of their systems just once. But then that ‘once’ becomes a friends-with-benefits arrangement, with one key rule: don’t fall in love.

But the thing about rules is that, when real feelings come into play, they’re impossible not to break.