Hi, if it's what your interested in then go for it!
I can't comment too much on job prospects but I'd imagine they'd be pretty good - civil service (diplomacy or Parliament schemes in particular), being an MP, working in the Government Legal Service, law firms abroad (you'd have to make sure you're qualified in whichever country you go to though) or maybe something like human rights law and advocacy in places where peoples' rights and freedoms are limited. You could also work for the United Nations or European Parliament but I don't know anything about how you would get into that.
Even if you can't get full-time work in the field, if you want to do this so much you can't imagine doing anything else then it's still worth studying at university and trying your best to get work experience, voluntary work and jobs related to politics or international law. If you never try then you'll never know where you might have ended up.
Loads of graduate jobs are open to people with any degree so you could still get a good job outside of the field if you were struggling to get into it, even if you just decided to do something else until you can find something you really want to do.