Al of the shows you refer to have their own websites, where you'll normally find a link for applicants. e.g:
http://www.bbc.co.uk/entertainment/mastermind/contestant.shtml
'Pointless' is produced by Brighter Pictures, which has now merged with its sister company (within the Endemol group), Cheetah Television, to form Remarkable Television. The Endemol website is currently only seeking applicants for 'Deal or No Deal' (via the Channel 4 website). That's probably because the next series of Pointless (to be screened in January) has already been recorded (or already has its contestants lined up) and no further series has yet been commissioned. If you want to get your name down for any future series, try contacting Catherine Bligh at Endemol:
http://www.endemoluk.com/?q=taxonomy/term/36/9&tid=8
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Chris