Transporting them is one thing, disposing of them is another. You need to check with where ever you intend to dispose of them because sometimes there are sometimes laws and regulations against such disposals, or if you are trying to dispose of them on private property, you could get someone upset.
The English Channel is a bit tough to find anything definitive. If you take a ferry, I am sure there are rules against just dumping stuff overboard in general. If you go to the shore, I have no idea, again probablly best to check with the locals. Don't forget (and I am not trying to be crude, but pratical) that most shores have a prevailing wind that goes onshore, not offshore, so if you were to try and spread the ashes at the shore, you might end up not getting them in the water, if you know what I mean.