B.o.B never came to the UK to promote Airplanes, yet it was one of the biggest songs of 2010, went to #1, and was released after the album.
Pitbull never came to the UK to promote Give Me Everything, yet it was one of the biggest songs of 2011 and spent 3 weeks at #1.
Chris Brown has been BANNED from the UK, yet he's still managed to have 5 big hits during that time (including Beautiful People, which was released after the album).
Eminem never came to the UK to promote Love the Way You Lie, but it was the best-selling song of 2010 (and also released after the album).
Maroon 5 haven't come to the UK to promote Pay Phone, yet it's being a massive hit, and been #1 twice already (and might get back there for a 3rd time this week).
There's so many other examples of this, but I'll stop now.
Everybody else gets big hits in the UK without setting foot in the country. Nicki has a massive advantage over all of them now that she's spent over a week here and performed at a load of festivals + done a tour.