You can use a jet wash or steam cleaner, just keep the lance a good distance away from the engine bay. You only want to use the water spray to clean and not the pressure of the water spray.
As better suggested, spray on a degreaser such as Gunk or Autoglym Machine cleaner, agitate it with a brush and then wash it off with a hose pipe set to sprinkle, but try and avoid spraying water directly onto the distributor.
Once you've finished, dry as much as you can with some towel, you'll probably need to pull the plug leads off and dry out the ends of the leads and the spark plug wells, then it would be best to spray some water dispersant such as GT85 or WD40 over all of the electrical connections in the engine bay. Lastly start the engine up to make sure it runs without any misfires and then let it idle for a while with the bonnet closed to dry everything off.