If given the choice between LSE Maths and Economics and UCL Economics which course would people choose? This is assuming you would enjoy a Maths and Econ course as much as an Econ course.
I would go with Maths and Econ from LSE, I know some people working in IB and some of them say that if they had the choice again they would have done pure maths instead of econ or at least a joint degree.
What do you mean it's hardly a rumour, it's certainly not a fact, and you told us something that you have obviously heard from someone else, thus it is a rumour.
The words came straight from mouth of the head of grad recruitment for said bank, so no it isn't a rumour.