Speak to the OU tech team before you buy it, OP, as some courses use certain software packages (for online lectures, submitting assignments, online library database, etc).
I'm sure most laptops/chrome books would be ok but I'd double check before shelling out.
Also, I think you still get an office 365 package as part of your course (Microsoft Word, Excel, PowerPoint, etc) so don't pay extra for this when you buy your laptop. Again, the OU IT team will be able to confirm this.
It's also worth checking how much of your course will be online - some of the OU courses I've done in recent years haven't had any printed materials and they've all been digital, which means you need to decide whether you are going to print everything out yourself (I think the OU will probably do it for you if you have SEND though) or whether you're comfortable reading from a screen. This might influence they type of laptop you buy and whether you need a decent printer too.
It's also worth thinking about whether you're going to be making notes on the laptop or doing them by hand, and what you'll do for tutorials (if your course has them).
I wouldn't spend too much at this point if you're just starting the course as (1) the OU has some helpful software that might make a difference to how you want to do things, (2) fellow students will share ideas about how they do things once you start the course and (3) it often takes some time to learn how you personally work best with studying and technology and you'll likely find that your IT needs evolve over time.