This was probably asked here before, but I didn’t find it in recent postings. I am an undergrad who initially majored in economics but my math is rusty at this point and for the sake of maintaining a good GPA for grad school I have decided to switch to Finance major. Now, how important will be the fact of not going through econometrics if I was to try financial mathematics? I am planning to finish at least Calc II and Linear algebra at my own pace, and may be more before I apply to grad school. And how overall Finance differs from Econ undergrad for Financial mathematics/risk management/portfolio management-analysis?

If you want to to your graduate work in math finance, econometrics isn't really a requirement. I would focus more on finishing your linear algerbra, pde's, ode's, probablitlity and Calculus (all levels available at you college) or skip the calc and take courses in Analysis.

If you go to the website of some MSc Financial Mathematics programmes in UK, you should able to find some admission tests for testing your mathematics background, too.