The largest escape attempt made by German POWs in Great Britain during WWII occurred at Camp 198 in South Wales, aka Island Farm, in March of 1945.

According to a website dedicated to the camp:

Less than four months after the first officer-prisoners had started arriving at Island Farm and two months after [camp Commandant Lieutenant-Colonel] Darling had discovered the first of the paired tunnels, 70 prisoners escaped.

The site has lots of great details, including:

• details and pictures of the escape tunnel
• a BBC news recording (00:40) detailing the escape
• Multiple news cuttings reporting the escape and recapture of the POWs
• newspaper article on the 55th anniversary of the breakout
• actual photos of the escape tunnel, re-opened in 2003