He also disliked the IRA connections, apparently, but smuggling via Ireland was easier than via France/mainland Europe. In the politics of the time, and perhaps from being Welsh, it was easier to excuse the IRA, I think a fair few Welsh and Scottish subjects of HM Elizabeth II would have had sympathies with the "cause", although not with the manner in which it was being fought for (violence versus Scotland's wait of 30-40 more years for a referendum; some Irishmen obviously wanted a resolution within their lifetime).
The Mr. Nice moniker comes from having bought a passport/identity from a real person by that name (Nice as in the French town), which he then traveled under and adopted. It was genuinely his name for a while.
MI6 was known to exist even it not confirmed by the govt. Ever heard of James Bond? Myth rooted in fact, every UK citizen who has read Flemming/Greene/etc knew there was an MI6.
Whatever you think of Marks, he does have a folk hero side that appeals to the British, someone who found his way into the establishment, but then rejected the banality of that life and instead lived a life most of us haven't the gall to even attempt. I think that is why he is appreciated by the British press.