Extracurricular activities aren't that important, universities in the UK are more interested in your particular commitment and awareness of your subject area. If you can undertake some activity related to your prospective subject area, such as Physics/Maths Olympiads, or science fairs (such as the Big Bang Fair) then that might be worth considering, but no university will expect that and outside of Oxbridge it's not even that common (and they specifically note that they are aware not all applicants will have had the opportunity to pursue those kinds of activities and so still don't expect them).
Doing some wider reading relevant to your subject might be worth doing e.g. over the summer though, and should be something most applicants can do reasonably. There are plenty of "pop" science books relating to physics which may be of interest which might be a good place to start. You may also be interested in some books on the history and/or philosophy of science/physics, which can help contextualise the development and "practice" of physics (or science more generally). The widely celebrated Feynman Lectures on Physics may also be worth checking out, and are available free online - these will give you an idea of how calculus is used in physics (as in degree level physics) and might help develop your physical intuition as well.
However don't let the above distract you from your studies as the main thing they are wanting to see is your ability to perform academically in relevant subject areas; ultimately wider reading etc, is just a side benefit secondary to you actually being able to understand and apply the subject material.