Well, neither is really ideal for investment banking, although KCL better than RHUL. Investment banking doesn't care what you studied, only where - that is, whether you studied at a target uni (and neither are target unis, although I believe KCL is commonly considered a semi-target).
Beyond that, they're very different courses. Both could lead you into data science or potentially software engineering (I know physics grads who have gone into both), although CS is more obviously related.
What do you actually want to study for the next 3 years? It's all well and good to plan for a career, but if you aren't going to make it through the degree to that then your plans aren't that well laid! Is there one you have a greater personal/intellectual interest in?