The degree requirement in a lot of countries is for visa, rather than teaching purposes. You could have a degree in biology, which wouldn't be terribly helpful in teaching English. If your a native speaker and have a TEFL certificate you shouldn't have too many problems satisfying your teaching obligations.
To be honest, I'd save the money you would spend on getting your modern languages degree, it probably wont return your investment in terms of enhancing your teaching skills. In your situation I would try to get a job in Asia with the qualifications you have. You could probably get a job in China, and need no qualifications whatsoever to teach in private language schools and government run schools from kindergarten to middle schools here, which is where the bulk of the work is to be had. Have a look here first though.http://middlekingdomlife.com/guide/
Assuming you own your own home, you could rent it out while teaching in Asia. You won't save much if any money on TEFL teachers wages, but you will have a better quality of life than you might have back in the U.K. at least until the recession is over.
I've met a couple of people in their sixties still living and teaching in China, but some schools have a cut off point between forty five and sixty. I seems to vary from school to school. The age of retirement in China is 55 for women and 60 for men, but it seems to be different for foreigners, as many things are. It's hard to get a working visa for China without a degree, but there may be other options. Have a look at the job boards at Dave's EFL Cafe http://www.eslcafe.com/
and maybe contact a few places to get a feel for what if any age policies they have.
Good luck with it all.
Be the change you want to see in the world.