Birkbeck Maths Graduate Diploma as a jumping board onto Maths MSc programs

Hi,
So for context I did Economics for undergraduate, got a 2:1, now I wish I'd chosen maths and want to do a maths MSc.

I've considered some options for brushing up on my uni-level maths and getting up to speed with what would be expected for a prospective Master's student and the best option to me seems to be the Birkbeck Graduate Diploma in Mathematics.

Can anyone confirm whether it's a good course? Is it a good jumping board onto more competitive maths MSc programs (e.g. at UCL, Imperial, Oxford, Cambridge)?