I got my degree ('mathematics as a double subject') over 30 years ago, so I'm not ideally placed to answer your question ;-)
However, you might find some useful information on the Prospects website. It's the official careers site for UK graduates. If you check out links such as 'Options with your subject', 'Use your qualification' or 'What do graduates do?', you should find plenty of information about possible career options:
http://www.prospects.ac.uk/cms/ShowPage/Home_p age/Careers_advice_main_menu/p!eikamj
Chris