Just so you know, there aren't any really 'useless' degrees, just useless graduates. A dear friend of mine will be on a (UK-based) banking management graduate scheme this year having just completed a degree in French and Spanish at a mid-range university. If that isn't 'useless' I don't know what is. **** graduates have **** prospects. Certain courses attract more **** graduates, that's all.
Chinese is a reasonable sell, particularly as, although you aren't Chinese and don't speak Chinese to the standard of a native, you are British educated and you'd speak Chinese better than 99 (.999..)% of British people. And you probably speak English better than 99(.999..)% of Chinese speakers too. Understanding of the culture and history is going to be helpful as well (indeed, a major concern of international employers is finding people who understand cultures and techniques of their destination country).
Regardless, you'll still be in a better position than most fresh graduates anyway, with several years' experience in a proper job. Even if you end up back in nonspecific employment it's no bad thing, and you might have a few more doors opened by the piece of paper.
In all likelihood, in three years' time you might have a totally different outlook anyway. Solipse's advice of trying out an evening course is excellent advice, but of course be sure that you don't waste degree funding options in doing so - pick cheap courses subsidised by local councils, etc.