If you know that you want to do theoretical physics, then I would say it is best to specialise in it from the beginning. There is a lot of theory and maths that you need to learn, and a theory degree is designed in such a way that you don’t have any gaps in your knowledge. However, don’t overdo it. I did a theoretical physics degree and managed to avoid almost all labs, but now I sometimes wish I had done at least some of those labs. A general physics degree is also fine, but then you have to think more about your option choices. Usually you can change courses within some limits, so as long as you are aware of the requirements you can still change your mind afterwards.