From the CAA RT Discipline Safety Leaflet
RT Discipline
Controllers should endeavour to limit the number of instructions
passed in any one transmission to a maximum of three - ideally only
two if practicable. Where there are large amounts of numbers to be
passed, then speak clearly and slowly.
So that's my best practice
In the airspace I operate there is generally no need to give QNH, but I regularly give a squawk and an instruction on first transmission where circumstances require it and permit.