This is one of those questions that haunts most people new to Crossfit in London, how do you get hold of the essential equiment that you need without being overcharged or ripped off.
Its interesting to note that Both Andy from Crossfit Reading  and Andrew Stemler from Crossfit 
London UK both mention and recommend  
Jordan Free weights as a Central supplier of Crossfit type stuff   in and around london, and indeed the rest of the country.
After that, Andrew Stemler, who occassionally imports gymnastic rings, always says, order direct form the USA, its cheap and easy, and there is a link below
BUY SOME  ELITE GYMNASTIC RINGS NOW!
     No one will take you seriously if you do not incorporate elite rings into your training. They are one of the better training tools around ( and have been for many years)
  
     
           
     DANIEL CRAIG USES RINGS
  
     
        
  
           We were delighted to discover that James Bond Star Daniel Craig has incorporated ring training into his training schedule and is close to mastering the Crucifix.
  Rings were also used by the cast of  "300".  

  Elite Rings are used By Crossfit trainers like Andrew Stemler to build fantastic upper body strength  
    BUY THEM DIRECT FROM THE US MANUFACTURER
     The Elite Gymnastic Rings are portable and lightweight