In my review of Writing for Television, I said: Grud, I wish I'd read this before I decided to spend a month writing a Doctors spec script several years ago! It wouldn't have made my writing much better, but it would have persuaded not to send a spec script for an existing show to that show. As a result, Paul Crilley asked this: Why would it have convinced you not to write a spec for Doctors? I've been considering doing just such a thing when I finish with current novel deadline.
In American TV writing, one way to break in is by writing spec scripts for existing shows. In his book Crafty TV Writing, for instance, Alex Epstein suggests writing two different specs - one for a character-led series like Gilmore Girls and one for a more plot-led or genre-heavy show, such as Battlestar Galactica or CSI. You use these speculative works to demonstrate you're ability to write for a show readers already know, showing you can script for other people's characters and concepts.
Chances are, your first job in TV will not be on a show entirely of your own creation. [Note: you should never send a spec of, say, CSI to CSI itself - that's bad form, apparently. If your goal is to write for CSI, do a spec for a similar kind of show, such as Cold Case. If you're into medical drama, you could spec House and use it to target Grey's Anatomy, as another example.] Leastways, that's what all the books and TV screenwriter blogs seem to suggest - mileage may vary.
British TV takes a rather more perverse attitude to spec scripts. It seems you should never write a spec script for an existing show. [Grud forbid you should send your spec script to the show you spec'd, like I did with Doctors - that's even worse. Of course, if my writing had been good enough at the time, I'm sure the script editors at Doctors would have forgiven my gauche stupidity.]
Readers want to see your original script, your original ideas, so they can assess your ability to plot and give good characterisation. Most of all, they want to see the evidence of your distinctive voice. Why? So they can hire you to write for an existing series where you'll need to sublimate that voice so you can write somebody else's characters. Hell, write for most soaps and you won't even get to write your own plots, they'll have been carefully handcrafted by the storyliners first.
So, if you want to write for British TV drama, don't labour over a spec script for an existing show. Create your own witty, original and exciting script instead and use that as your calling card. With enough talent, luck and persistence, you might be given a chance to write for an existing show - and get paid for it.