Imperial is by far the best for Physics, followed slightly more distantly by Durham. UCLs Physics course isn't bad per se, but it is nowhere near the level of either of those (unless your specific interest is in planetary science or astrophysics - particularly, NOT high energy astrophysics - which it has some specialist expertise in).
NatSci at UCL depends somewhat on the courses you pursue - I'd say overall Durham's NatSci course is better though, perhaps excepting if you want to pursue earth sciences/similar (which are better at UCL on one of their single subject courses anyway, as the NatSci course doesn't seem to facilitate it that well).
KCL's Physics course is better than UCL, for reference. You should stop looking at rankings and look at the course content (and former students comments/destinations).