Starting with a disclaimer, I've not done much research into it so I'm going with everything that I'm aware of. For international fans and any other UK sites that I've missed out, drop them in the comments below to help each other out!
All episodes available via subscription only on BritBox.
Also available on DVD from main retailers.
Most episodes if not all can be found on Dailymotion too.
NEW WHO
All episodes available for free on BBC iPlayer.
Series 1-10 available on Netflix UK.
Also available on DVD from main retailers.
Most episodes if not all can be found on Dailymotion too.
THE SARAH JANE ADVENTURES
Not currently available via any streaming services.
Also available on DVD from main retailers.
Most episodes if not all can be found on Dailymotion too.
TORCHWOOD
All episodes available for free on BBC iPlayer.
Also available on DVD from main retailers.
Most episodes if not all can be found on Dailymotion too.
CLASS
Available on Netflix UK.
Also available on DVD from main retailers.
Probably can be found on Dailymotion too.
For more, there's books, Titan Comics, Big Finish audio productions as well as plenty of fan-made stuff on the web too!