How important is the bsc msc decision when choosing a uni course

[deathalele]

I'm looking at studying physics/maths at uni and want to get a masters (or further).

lots of the courses i've looked at offer a choice of bsc or msc degree, but some of the interesting ones only go up to bsc.

 

Basicly if i get a bsc how easy is it to then get a msc in in a related subject with another year or two of study?

 

thanks in advance