WITH US megastars like Lady Gaga, Katy Perry, Beyonc and Rihanna hogging all of the best seats, there hasnt been much room recently for a globally-successful British music phenomenon. But then, wouldnt you know it, just like buses, two of them rock up at once. First, squillion-selling Adele, and now fellow BRIT School graduate Jessica Cornish, aka Jessie J. Watching Jessie , 23, sending 75,000 music fans bonkers on a sodden Sunday evening at Londons Wembley Stadium for Capital Radios Summertime Ball, its remarkable to think that her first single, Do It Like A Dude, was only released on download last November. The Sun