There is definitely a common stereotype of Nigerians in the UK as being associated with crime. (I have to plead guilty to posting on AB plenty of times relating to, say, 'a nice Nigerian prince who has kindly offered me a very large sum for allowing him access to my bank account'; so I've not only acknowledged the stereotype but probably reinforced it as well).
That stereotype isn't only one held by Brits; I've worked with several people from various African countries who've made it extremely clear that they don't want anyone to confuse them with Nigerians because of the stigma they see attached to that nationality.
So perhaps President Buhari is doing little more than acknowledging a stereotype of his compatriots? After all (whether it's true or not) most Brits recognise the stereotype of ourselves as reserved and unemotional, whereas most Germans would recognise a stereotype of their compatriots as being efficient but humourless. (At a more local level, Liverpudlians might recognise that they're often thought of as extremely friendly but also extremely light-fingered!).
So, as I see it, there's no real 'news' in that report, which seems to make no reference at all to the rumours that, while President Buhari is officially in London to attend the Supporting Syria and The Region Conference, he's actually here to seek urgent medical treatment. (Those rumours have gained credence by the fact that he's just handed over power, on a temporary basis, to his vice president by filing an official notice that he's 'on vacation').