Your insulation works best if it is not compressed. The National Physical Laboratory (NPL), the premier testing agency in the UK,  tested insulation when it was compressed. The results were worse than anyone thought; if you squash your insulation from 270mm down to a joist height of 100mm then its performance reduces by 50%. If you squash it down to 75mm joists, it loses 60%. So the heat lost at least doubles. See NPLs press release.

Also when compressed there is no gap between the item compressing it and the insulation. This can lead to damp. ​

An important element to consider when boarding a loft is can my joists take the weight? For example, most new houses are constructed with truss roofs and the British Standard for these (BS5268) says that the joists must be able to carry a permanent load of at least 25kg per square metre (e.g. for storage) plus a temporary load of 180kg (assumed to be one person walking around). Every new house has to be built at least to this minimum legal standard, though many can in fact accommodate much greater loads. Older properties vary as would be expected, some are much stronger than this, whilst others are not. If you are in doubt, please seek advice from a structural engineer. ​

Also, if items that require periodic maintenance (for example solar pumps and inverters, alarm systems, water tanks, mechanical ventilation systems or gas boilers) are installed in the loft, it is important that a minimum access requirement of a walkway from the hatch to the unit plus a 1m2 working platform is provided. An employer cannot legally send his or her staff into a loft to maintain equipment without a safe working access path; And a fixed ladder.