Supercurricular Reading - Have I messed up?

The only universities which are going to care what wider reading you've done, if any, are Oxford and Cambridge - and for Physics it's really not that important, compared to your actual academic achievement in mathematics (as the primary concern) and physics (following that) to date (including any assessment tests). I certainly can't see that they are going to care you read "A Briefer History of Time" vs "A Brief History of Time".

You might want to read through (some of Volume I of) the "Feynman Lectures on Physics" (available free online I believe), which is actually...physics (although it's not really going to teach you to solve exam questions in physics, it will help develop your physical intuition and in typical Feynman style is very readable). Ultimately though unless you're applying to Oxbridge, I really doubt your "wider reading" will matter at all, and even for those universities it's more important you can actually understand and solve mathematical and physics problems (which is what the interviews at Oxbridge actually entail, as far as I can understand - there seems to be little to no discussion of anything else and it's mostly just for them to see how you deal with applying your current knowledge to unfamiliar questions).