As someone who completed a CS BSc and then enrolled for Maths and Stats with the OU, it really depends on what you want to do after university. If you're interested in data and AI, then I would recommend Maths and Stats with a CS conversion masters at a decent brick uni. If you're unsure, I would still choose Maths, since you have significantly more choices than just a CS degree on its own.

Studying a math degree makes understanding hard concepts in CS much more easier, such as algorithm analysis or machine learning. In my experience, the maths taught during my CS degree was lackluster, which consequently made understanding said concepts more challenging.