I'm sure there are plenty of opinions on whether LBS is "good". I'd say it is good - certainly the most prestigious in the UK, and in the "world class" category.
It will definitely help you get into excellent companies, assuming of course the rest of your background is suitable. WSJ is the main league table to be based solely on how much recruiters like different business schools' graduates - LBS came fourth internationally.
According to the WSJ breakdown, the top three employers at the last graduation were McKinsey (12), Barcap (12), and BaH (9). So that's 33 out of a cohort of around 325 fulltimers even before taking into account hiring by smaller but no less prestigious firms.
Most b-schools allow you some choice of classes so you can develop an academic concentration and companies will expect your choices to be in line with your application e.g. if you are going into banking, you should have taken a good proportion of the finance classes available and performed well in them. In that respect, grades do matter.