Hi
You need to be quailified first to use a cradle either by IPAF (International Powered Access Federation) or the CITB (Construction Industry Training Board) or someone similar first.
For work, just trawl the internet for companies who do cradle work in the London area and then phone them.
Also, I forgot to add this, if you do do cradle work use a harness with a fall arrester as an added safety precaution but remember it does not work below about 15 ft.