I was looking for contact info on the TOTP web site, thinking of writing to them about the lack of live performances, and I found the following "official" response to the miming issue:
Why do so many acts on the show mime?
Chris Cowey, the show's former Executive Producer says:-
"We always ask for a live performance or a live vocal in the first instance. There are often several reasons why this is not possible:-
Increasingly, the production of songs in the studio means that the talent wouldn’t be able to reproduce the sound of the recording when performing live.
Secondly, there might be an exhausting dance routine which would mean that they wouldn’t be able to perform and sing to the best of their ability.
Thirdly, if you’re a pop star, your schedule is immense. You’re doing every radio phone-show, TV appearance, magazine interview that there is, and so by the time that they get to TOTP, they may not have the time to rehearse and do a live performance.
So, do we stand hard and fast and say "No, you’re going to sing live" or, do we look at the performance and say "Ok, we understand why it would work best if you mimed". Each act is rated on its own merits.
Also, I think that there is a lot of misconception. Some artists are victims of genre, where people say "Oh, they can’t sing", but this isn’t the case. Steps sang live twice, Westlife have sung live on the show 3 times and when you go to the major bands such as Coldplay, they never mime. David Gray mimed once but that was because he was very ill."
So I guess we're stuck with the miming. How infuriating.