I have a gas hob to be fitted in the worktop of a standard fitted kitchen with laminate worktop. Underneath is a 3-draw cabinet with 60mm pipe space behind.
The connection from the 22mm supply will be 15mm copper rising vertically behind the unit in the pipe space, a right-angle elbow into the cabinet and copper running just below the worktop for an inch or two, then a 15mm x 1/2" iron elbow connecting to male bsp taper on the hob.

What is the usual procedure to make the connection, since there is no access behind the cabinet when the worktop and hob is being fitted ?

There's only a couple of inches in which to make the connection below the worktop, so it seems that the pipe would have to be positioned very accurately before the units are fixed in place.

Cheers,
Simon.