You will have to clarify "below ground level" is the vehicle being driven in to a trench at the side of the work site or being used from a road to lift to an elevated work platform e.g. on a hill side.

As with all equipment there are different designs dependent upon intended purpose - most commonly associate the generic term HIAB with the fixed / extending boom loaders preferred by construction supply companies where the intention is to off-load from the trailer deck and lower to ground level at site. There are more complex craning solutions but these are less visible on UK roads.

What does the drivers training / manufacturers operating manual state?

The appointed person should be developing a lift plan comensurate with the specific requirements of the lift and incorporating the following British Standards:

• BS 7121-1 2016 Code of Practice for Safe Use of Cranes 
• BS 7121-4 2010 Code of Practice for Safe Use of Cranes Part 4 Lorry Loaders

Readily understandable rated capacity charts should be prominently displayed to the lorry loader operator. Rated capacity charts for special applications should be available in the lorry loader operator’s handbook. The lorry loader should not be operated outside these parameters.

The standards advise to avoid working on sloping ground due to the increased load placed on the equipment (risk of tipping) and the same would apply to loads lifted from beneath unless the lift plan drwon up by the competent appointed person following all the information above can confirm it is safe to do.

I hope that helps.