Funny that because 50's are the ones you normally get most grief about from shops/pubs etc.
Let see if this works ..... it didnt, follow the link http://www.bankofengland.co.uk/banknotes/pages/about/counterfeits.aspx but theres about 4 times more fake 20's than 50's out there
Counterfeits in my experience are nearly always £20's or £1's like Rich says, and once youve had some they arent that difficult to spot either
According to this there's £52,520,000,000.00 worth of genuine notes in the uk. That's so cool