There is sadly quite a lot of misinformation on this thread, and I've seen people on YouTube recently believe what's been written. To clarify, there absolutely IS a need to leave a gap between the top of your insulation and the underside of the boards.
The British Board of Agrement (the BBA - UK's authority for approving building products) says this should be 50mm (for example, see Section 8 on the approval certificate for the LoftZone raised loft boarding product here:
http://www.bbacerts.co.uk/CertificateFiles/52/5269PS1i1.pdf) though I've seen a report from the Buildings Research Establishment (BRE) that says it can be only 29mm if you have excellent ventilation from eave to eave.
The point of the gap is to remove moist air. How much you have depends on how humid your house is and how well sealed your ceiling is, so not all houses will suffer this, but water vapour moves through the plasterboard ceiling, even if there are no gaps, and will cool as it passes through the insulation. So there's a risk that it will condense on the underside of the boards, causing damp and mould, which is why the BBA and BRE insists on the air gap. It's particularly bad in the winter, when the loft is cold. I've seen plenty of cases of boards getting so sodden that both them and the insulation have to be thrown away, and where fungus/water stains the ceiling and leaks down into the room below.
As a DIY'er it's up to whether you want to take that risk, but it wouldn't be allowed for a professional builder. There are plenty of ways to raise the boards above the insulation, it's not hard to do.