Loft Insulation Alternatives (UK)
You'd need to double-check, but does the 270mm regulation apply to existing houses as well? Otherwise, I've seen plenty of lofts cross battened with 150mm or 175mm deep timbers and boarded on top. Not sure where you want the air gap, condensation shouldn't be a problem here unless you plan on not heating the office...
An alternative is to go for a warm roof by putting the insulation in between the rafters instead (where you do need an air gap).