As I said somewhere higher up in this thread, this reflects a confusion about what the Mercury is supposed to be for. It’s not meant to be a prize for new acts yet they consistently nominate lots of debut albums, many of which are really not that good. I’ve nothing against promoting new acts, but they tend to detract from the idea that this is a prize for the ‘best’ album by crowding out more established acts.
Personally I think they should restructure the whole thing and have a prize for the various genres (best Pop album, best Alternative album, best Jazz album, best hip hop album etc) and maybe one for best debut album. They could have (say) a three album shortlist for each of those prizes, then choose winners and put all of them to the jury for the overall prize. That would satisfy the promotional purposes of the prize and also give more chance of picking a ‘best’ record.