You simply write to your employer. We suggest the following text:
"Dear [HR representative]
Further to my conversation with [your line manager's name] on [date], I hereby confirm that I am withdrawing my agreement to opt-out from the weekly working time limit set by the Working Time Regulations. Specifically, I no longer wish to work more than 48 hours on average each week because [insert reason, e.g. it is having an adverse impact on my health].
I would ask you to acknowledge this in writing and to let me know when this will come into effect. I look forward to discussing how my hours will be adjusted to take account of this.