I think (but am not sure) that LSE guarantees stipend for all 5 years. I think it is about 22k GBP and tax exempt and you can do summer RA or summer teaching to earn more. UCL doesn't provide any funding for the MRes year (year 1) and it looks like not everyone gets a full tuition offset either. But from year 2, you teach and get a stipend, which I think is about the same as LSE.
Out of curiosity, do you have offers from LSE Econ and UCL Econ? And if so, are you considering UCL? I thought LSE would be a clear pick in terms of the department? Although UCL is pretty good too!