This is the Song For Europe model of the last 60s and early 70s. Probably the most successful time in the UKs Eurovision history.
It coincided with the light entertainment trend of 'personality' specials lead by singing stars; Lulu, Cliff Richard, Rolf Harris and Sandie Shaw. Household names could be signed up knowing that at the very least they will get to represent their country and no go through the indignity of not winning a UK selection process.
Perhaps the BBC thought that when the competitive aspect of UK selection was removed following the second 'Your Country Needs You' that it could attract big stars again based on this principle. But the draw of having your own BBC1 special dedicated to you in which you get to sing 5 new songs - in which you were the guaranteed winner - was no longer in place.
I have no doubt that the BBC could attract, at first, someone of Pixie Lott level if the gig was guaranteed to be hers. And if an hour long BBC1 weekend music special was part of the deal, I suspect her agent would give her little choice in the matter.
They also did this in 1992-1994 with Michael Ball, Sonia and Frances Ruffelle and I always thought it was the best way of doing it but these days it seems received wisdom is TV audiences wouldn't have the patience to sit through 8 songs of varying quality by the same act.