I'd definitely support the idea of writing a few episodes to get to know the characters first, before submitting anywhere.
My preference would also be to submit to production companies rather than BBC writersroom (and not because they rejected me... ok, a little bit - but mainly because they appear to receive an overwhelming amount of scripts and never seem to progress with them)
http://www.chortle.co.uk/news/2007/03/16/5132/the_comedy_cafe
Work has started on a new sitcom from Pulling creator Sharon Horgan.
Angelo’s is a six-part series set in a London greasy spoon café, due to air on Five later this year, and casting details were announced today.Horgan will star as a police officer, desperate for a baby with her husband Dave, played by Paul Garner. Dennis Penis star Paul Kaye also appears as Dave’s sleazy brother Kevin.
Other stars include Steve Brody as the café owner, Shelley Longworth as his 19-year-old X-Factor wannabe daughter, and Alice Lowe, of the Garth Marenghi team, as her best friend Alicia; while customers include mime artist Kris, played by Simon Farnaby, and nymphomaniac minicab driver Shelly, played by Miranda Hart.
Production companies can get put off if there are similar sitcoms out there.