Was there ever an official answer why the BBC abruptly stopped offering people any say on the the song/artist choice? I've heard lots of theories (the accompanying programmes too expensive, the public always choosing awful acts) but not sure I've heard an official reason. Having grown up with the Song For Europe-type setup it still seems weird to have them seeing "here's the song you're getting" - kind of loses the investment the viewer had in the song had before.
I think the issue is a mixture. One-off shows are always expensive - as there is no economy of scale - and historically the selection show hasn't rated that well as a one-off. Not many channel controllers want to commission an expensive show that doesn't rate well...
To be honest, given the low ratings for previous selection shows, I'm not sure the wider UK viewing audience DID feel that they'd invested much in the selection of the song. Eurovision itself rates much higher than most selection formats after all.
The most successful selection format we've had was the Graham Norton/Lord Lloyd Webber format - which was commissioned as much to cope with the lack of a West End show to cast as it was to find Eurovision entrants. (It took the slot of Maria/Joseph and used a similar format - and I think was used as a quick replacement when they didn't have a West End show to cast?)
The UK music industry is so radically different (apart possible from Ireland's) from the rest of Europe, that we simply can't compare how the UK selects its entry (which will never be a mainstream, current, successful chart act with label support in the UK in the current industry) with how countries like Sweden, Norway, Malta etc. select their entry.