@Series B notes
You need to have a look through the web site and read about the notes, visit the A Series Types page
to familiarise yourself with the characteristics of each Type.
There were no security threads on the higher denomination notes. Also, there are no known instances of £50 or £100 contemporary forgeries. The only forgeries of these notes have been recent using colour copiers/printers, so called 'replicas'.
The note in th epicture is genuine.