Physics isn't always a requirement for Medicine, it rarely is. Doing Maths instead of Physics is a perfectly viable option, especially considering you do a Statistics module, which is sought after and highly commended for Medicine. Biology and Chemistry are also a really good couple, as Chemistry helps you understanding bits in Biology.
Chemistry is harder than Biology, but as long as you're passionate about your future and have ability do well. You should be fine, there's plenty of support at school and online and I have no doubt you'll be great.