Air is the insulator used in loft insulation. The thicker it is the more air
is trapped and the better insulation.

By boarding the loft, you automatically trap air under the boards, and
provided that air doesn't circulate, I would think that there is no
advantage in providing a thicker layer below the boards. In fact if you
sealed the gap between ceiling and board completely, you wouldn't need any
fibreglass, the air would act as the insulator as in double glazing.

The fibreglass you have there will impede the air circulating and so I would
just go ahead and board over, paying attention to general loft ventilation,
especially over the bathroom, as discussed.

Colin

PS Don't forget to box in any halogen downlighters if fitted