A fascinating glimpse into the "shades of grey" over the legality and illegality of fighting in Syria, for UK nationals / residents. Under the headline:
Syria conflict: British jihadists are nothing like the freedom fighters of the Spanish Civil War
Prosecutorial discretion:
For example, let’s imagine a Briton is visiting family in Syria and the neighbourhood comes under attack by regime forces, or indeed rebel ones. If said Briton were to subsequently use weapons to help defend family members or other innocent people, he would not necessarily be prosecuted in the UK. However, those who join terrorism-linked groups and proactively engage in militant activity are far likelier to be prosecuted.
Link:http://www.telegraph.co.uk/news/worl...Civil-War.html

I suspect discretion was used when many exiles returned to Libya, to my knowledge no-one was arrested.