In a recent ask Reddit thread someone post
"In World War Two, MI5 captured or turned every single German spy sent to Britain. Not some, not most: All." he cited this book as his source
The Double Cross System in the War of 1939 to 1945 (1972) by John Cecil Masterman
I was wondering whether this claim actually has any fact to it but also in a more general sense how good we the British at catching people within there own system that had turned and were supplying information to the Germans or spy rings in the country
You should also look up "Agent ZigZag" and "Double Cross" both by Ben Macintyre which covers the turning of double agents by British counter intelligence during WW2. Basically the claim is true, as the Nazis attempts to plant spies in England were pretty much a complete failure. German intelligence was beset by lack of experience in many areas, infighting amongst the multiple different agencies, rivalries, and of course purges by organizations like the SS. There are recorded incidents of German spies landing in the UK being captured almost immediately because they didn't even speak English. British counter intelligence often used these would be spies in the double cross system to gather intelligence or send false intelligence, including to help in the misdirection of where the real D-Day landings would take place.
Yes, it's entirely true.
It was mostly done through a man called Arthur Owens, who took on the name Johnny O'Brien and told Germany that he was a British spy and offered to double cross for them. Any German spy sent to the UK was told to check in with him, at which point MI5 would propose spying for Britain. If they refused, they'd be arrested.
British intelligence was sent on to the Russians who used it to their advantage. It was also Britain that cracked the enigma code and ran a network of spies throughout the Third Reich that allowed the Allies to counter each move made by the Axis before it was made.
Operation Mincemeat is a fascinating example of what the British did as far as counterintelligence. Masterminded by the Twenty Committee (XX or Doublecross in Roman numerals), they basically found a corpse with no living relatives, a man named Glyndwr Michael, created an identity (including a fiance'), of Major William Martin, Royal Marines, dressed him as a sailor, and left him on the coast of Spain, where German intel would find him and his false orders.
This helped convince the German high command the Greece was the next target, when Sicily was the actual point of attack. Germany fell for the ruse, and while Sicily was no easy stroll, the Allies did encounter less resistance due partially to this ruse.
Additionally, The BBC being a state-run communications system often relayed messages to resistance forces and other people either hidden in scripts of their regular broadcasts, or coded messages on their foreign services in continental Europe. Some of these were undoubtedly designed to misinform the enemy.