I thought a tax exile was someone who moves their permanent place of residence abroad to avoid tax (whilst often contriving to actually still spend most of their time in the home country). If Mo wanted to be a tax exile, he'd be living in Monaco and paying far less tax. His situation is no different to the thousands of Brit ex pats working in the States. He's resident in Portland because that's where he has agreed with his employer is the best base to peruse his occupation.
FWIW he was not brought up in Somalia. He was born there but then lived for a few years in Somaliland, which, although not recognised by the UN has all the features of an independent state (including it's own currency). He lived the majority of his time in Africa, however in Djibouti (his Grandmother is from there).
Trust me, I live and work in the area of London where Mo grew up. That is not a fake accent. He is a Londoner through and through.
If he had been born in Australia or Canada or Italy but lived in the UK from the age of 8, this question of his nationality would simply never arise.