An LSE degree is respected no matter what you study, I wouldn't worry about the 'risk' of picking one subject over another. Arguably you're taking a greater risk by applying for something you don't enjoy.
And note that if you take LSE's BSc finance, you will still have to do all the compulsory econ modules, i.e. macro/micro, econometrics, stats etc, so it's already a good mix between econ/finance anyway.