Degree Selection Advice

I am hoping this is on-topic here. Profuse apologies if not.

So, I have a somewhat unusual situation.

I am looking to return to school after a long absence and, as I am only able to return to study in the evening, I am limited to one institution that offers evening degree courses in my area.

What I'd really like to study is Cosmology, but this University (Birkbeck University of London) does not offer a physics degree program. What it does offer is a Mathematics program, so my idea is that I can take that degree and then go for something more Cosmological at postgrad.

My question is how useful is a Math degree going to be to me if I want to work in Cosmology. Does anyone know of anyone who started out doing math and then ended up in the Cosmology field?

Also, does anyone know of any other Institutions in the UK that offer evening physics degrees?

Any and all advice gratefully recieved. :)