There are two main types of dual controls for driving school cars. Cable operated and rod operated. Cable operated duals seem to be becoming more prevalent. An advantage of rod operated dual controls is that you can have them set up either to work independently of the main pedals or in tandem with them. Some instructors like rod operated dual controls that where the instructors pedal moves at the same time as the pupils. The danger is that if you are not careful you can get your foot trapped under the pedal when the pupil operates the pedals!
The main supplier of dual controls in the UK is He-Man. He Man Dual controls are based in Southampton but have agents throughout the country.
List of He Man Dual Control Fitters
Swift 020 8650 8711 (London)
Car Controls Ltd 07968 073955 (Scotland)
City Motor Services 029 2036 4800 (South Wales)
Dave Bates Controls 07771 523009 (Bristol)
Dual Control Services (Dunstable) 07721 517513
Dual Control Services (North East) 07759 833932
He-Fit 01204 700625
Howes Controls 07718 731934 (Lancashire)
L D S Ltd 07989 182057 (Midlands)
Mobility Conversions 0161 745 9737 (Manchester)
T V M Ltd 07899 834559 (Surrey)
Stephen Cooper 07974 377801 (SW Wales)
DC Adaptions 07795 320771 (Lincs/ East Yorkshire)
Chadwick Auto Services 07917 061438 (North Wales)
George Cowan Scotland 07900 216496