Re: Billie Piper not involved in 50th anniversary
Well as others have said a UK Season can be as little as 3 episodes (Sherlock), 6-8 (Red Dwarf), 14 (DW), shows like Casualty which seems like it is on almost every week and the soaps which run multiple episodes per week all year round.
Shows like Casualty apart from the cast size (as mentioned above) also have little requirement for the building of new sets for each story or special effects which would save them a big chunk of change.
Gentlemen you can't fight in here - this is the war room.
Pres. Merkin P. Muffly