Under permitted development, assuming that your property is not listed or within a conservation area, the maximum allowed height is typically 1m including railings. Should you wish to go higher than 1m, a householder planning application would be required.
The critical issue is that you should be able to see pedestrians, especially small children, on the footpath as you exit your driveway. In addition to a consideration of the most appropriate height, the degree of vision splay from the drivers view is important. I suggest that you check the guidelines issued by your relevant highways authority.