On the whole you're right, but I don't think Dizzee Rascal has been booked since his DV conviction.
Slowthai was also quietly dropped from the lineup one year.
Aged 16 having a dad who can get people i assume free tickets for the 1975 must be worth a few social points!!
Re your last paragraph, they come and go all the time. I wouldn't be remotely surprised if they did it as a full one one-off.