I concur with Paddy B and suggest you obtain a copy of PM28 although it is somewhat contradicory and is a bone of contention in the UK and has never been fully resolved when a cage can be used and what for. Basically if you use a cage I suggest that it must be approved by your H&S people. Only this week I saw two guys try to free a rear roller door on a lorry by standing on raised forks! You could also contact BITA who are most helpful.
I worked for an UK attachment company for 20 years as a service engineer and manager. We manufactured and supplied saftey cages that were intergrally fitted into the truck traction electrics by the use of suitably positioned fail safe switches that disabled the truck unless certain peramiters were met (height, handbrake on etc) and the operator(s) in the cage pressed two handed buttons. The cages are better suited to electic trucks as they immediatley start and stop the electrics, whereas with engine trucks there is a delay in stopping & starting the engine. I also designed an intermediate switch controlled cage i.e. a basic cage with cut out switches. Please contact me for more detail. 07500 958698