There's only a few companies in the UK that sell "legal" stilts, Including Pro-Jump (us) PoweriSer, PoweriZer, 7leagueboots, fly jumpers and UKGoodDeals (us) who stock Powerskips.
All vary is price, there's a huge debate on what the "best" brand and what you can and can't do on them.
In terms of price: Pro-Jump's (same as Fly Jumpers or PoweriZers) sell theirs at £135 delivered, PoweriSers incoparate a cuff insted of a knee bar at about £200 and 7leagues are around £240, PowerSkips are £699 to £750 depending on whether you go with the fixed or moving footplate.
As this is mainly forum run by Pro-Jump, theres not a lot of discussion on brands other than Pro-Jump or PowerSkips, the following threads have some debate thou:
Need AdviseGetting New StiltsWhat brand to buy