TV YouTube is awash with legally dubious uploads, but some companies, notably BBC Worldwise who upload whole programmes themselves for promotional purposes.
All3Media is another one.
Find on their YouTube channel whole drama series of varying vintages most of which haven't been released onto the home market or subscription services. Here they are with descriptive quotes from Wikipedia because I'm being lazy:
"The Palace was a British drama television series that aired on ITV in 2008. Produced by Company Pictures for the ITV network, it was created by Tom Grieves and follows a fictional British Royal Family in the aftermath of the death of King James III and the succession of his 24-year-old son, Richard IV, played by Rupert Evans. It also stars Jane Asher and Zoe Telford. The series was filmed on location in Lithuania in 2007 and broadcast from January to March 2008. It was axed after one series due to low viewing figures."
Serious & Organised doesn't have a Wikipedia page. The YouTube page says: "Martin Kemp in his first series role since leaving Eastenders, stars as a detective in the Serious and Organised squad. The National Serious and Organised Crime Unit is charged with dealing with the most dangerous and professional criminals in Great Britain: crime families, Triads, gangland killings, extortion; major drug suppliers."
"P.O.W. was a television series consisting of 6 episodes, broadcast on ITV in 2003. The series starred James D'Arcy and Joe Absolom. The drama series is based on true stories, set in Germany in the year 1940 and follows the character of Jim Caddon as he is captured after his plane crashes during a bombing raid over Normandy. In contrast to previous entries in the World War Two prison escape genre such as The Colditz Story, it concentrated on escape attempts by other ranks rather than officers. The series was filmed in Lithuania and first broadcast on television on 10 October 2003. A second series has not been commissioned, though ITV followed it with several other World War Two dramas including Colditz and Island at War. The title "P.O.W." stands for "prisoner of war"." Note: the above playlist needs editing a bit. Have emailed them.
"Rose and Maloney is a British television crime drama starring Sarah Lancashire and Phil Davis as Rose Linden and Maloney, two investigators working for the fictional Criminal Justice Review Agency. This agency takes on claims of miscarriages of justice, assessing whether there are grounds to reopen old cases. Rose is brilliant but strong-willed and sometimes reckless. She likes to follow her instincts and play hunches and often comes into conflict with authority. Maloney, although Rose's superior, usually allows himself to be led by his more passionate colleague. Maloney is a by-the-book man and a little grey. He finds working with Rose dangerous but addictively exciting. A pilot was first broadcast on ITV on 29 September 2002. A series of three stories followed in July 2005."
Guardian The Queen. From YouTube: "For the very first time cameras have been allowed behind the scenes at the royal palaces to see the historic and hidden world of the Grenadier Guards."
Again, from YouTube: "WILLIAM'S WOMEN takes a light-hearted look at the world of royal dating. From the plusses to the pitfalls the programme takes a look at an area that is fraught with complications."
All3Media list also has loads of other channels listed here, but most of them just contain clips or Gordon Ramsey. Or both.