Dear Gilles,
BP stands for Bletchley Park, more formally known as the Government Code & Cypher School (GC&CS). Its most famous work was the decryption of German secret radio messages which had been encrypted using the 'Enigma' and 'Lorenz' cypher machines. The British codename for the intelligence based on these decrypts was 'ULTRA'.
Nick Beale is, of course, one of the foremost experts on the subject of 'ULTRA' and has provided a photo of the top half of the page from the following filed held at The National Archives:
Reference: HW 5/427
Description: German section: reports of German army and air force high grade machine decrypts
Date: Jan 25 1944-Jan 26 1944
https://discovery.nationalarchives.g...ils/r/C4303993
Regards,
Dan