It's entirely possible Frost became a fan of the franchise later on.
Or he needs the money and is doing typical PR. Either/or.
Pegg was wasted. He clearly jumped at the first role he could get, and he ended up in "The Long Game."
Reportedly, he was supposed to play Rose Tyler's dad, but scheduling got in the way, and he ended up playing the Editor of Satellite Five instead.
I strongly suspect the Christmas special is Coleman's last episode.

I though Who aired Sundays?
Regular episodes have been early Saturday evenings here in the UK since the relaunch, with 'specials' like the Christmas one's on whenever that falls.
And the preview for "The Caretaker" has a guy that looks like Matt Smith....with a bowtie too!
I wonder if 12 will make a remark
From TVTropes
And the preview for "The Caretaker" has a guy that looks like Matt Smith....with a bowtie too!
I wonder if 12 will make a remark
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