Miku English is on the way, as I have mentioned before.
Below is a short demonstration track: part of Coming Together, which was written by someone named as 'bsc a.k.a. kuni', which doesn't help me much(!)We see the notes on the editor, and we have the lyrics in case of need. Commenters so far (this was uploaded just yesterday) have been mixed on that point: some have been able to understand without looking at the words, others have needed them here and there.
Therefore it isn't perfect, but my trained ear suspects that this is at least partly down to the producer being non-English and therefore not necessarily knowing the exact pronunciation. Combined with what no doubt is an attempt to make it appeal primarily to Americans rather than Brits like me, it ends up with a slightly odd and very occasionally clipped accent.
Once producers in the UK get their hands on Miku English (which also sees the porting to Vocaloid 3 from her original V 2, alongside the Japanese original) and learn how best to tune her for 'proper' English pronunciation, I think we shall find some really good material appearing. The sheer clarity of her voice in this demo gives a strong indication that the potential really is there.
Perhaps this is what we need to open up the Vocaloid world to a largely unaware Great Britain...