The assumption is that Britain regarded it as illegal in the same way that the USA would regard it as illegal if, say, the state of Texas was to declare independence today. That said, it's apparently a hot area of debate among people who study law:
On the other hand apparently the Supreme Court disagrees with me [1], and only 33% of people polled think that Texas can secede [2].
[1] http://blog.constitutioncenter.org/2012/11/constitution-chec...
http://www.wnd.com/2012/11/secession-its-constitutional/
It's not because there was the Civil War that the Civil War outcome was Lawful. The Southern States had the right to secede and that right was not recognized by Lincoln.