I think every country has their own system of assessment. However, as far as I know, ABRSM, trinity and LCM are the most popular diplomas? (there is one in Australia, can't remmeber the name of it). In Hong kong, nearly 100% (of maybe its 100%) of employers / people would have heard of ABRSM and Trinity. So i would say these two examination bodies are recognised all over the world.
It is always a brilliant idea to equip yourself with some letters after your name - as many as possible. You may not need them afterall, but they may, on the other hand, get you a well paid job. I would say if you have confident in passing and few pounds spare (well..), then go for diplomas.
I am still only a student (and I am young indeed!
) - so I am not entirely sure what the employers are really looking for. Probably a good degree/well qualified, then your experience in the field etc etc. So i would say a good degree and experience are both very important, and this is why all undergrads in my subject have to do internship. good luck with ur decision.