I'd think it will need a 2.1 in an undergraduate course with some mathematical content. Can I ask what you background is and what you're looking for? I've not been to LSE so I don't know anything specific. However I've studied an MSc OR at another uni and did my undergrad at another which is popular for OR. Therefore it may be a biased view but from what i've heard the OR course is not as well regarded as say Southampton, Warwick or Lancaster but it's up there. However it is a more internationally well known Uni so it comes down to what you want the masters for.