It would seem that the original ceiling joists here were only 75mm - if they cover any span then there is a danger thew will sag and distort under the weight of just the floor boards - never mind anything placed on them. Hence thickening the joists should help. The cross beams I expect were designed to spread any point loads on the floor over several existing joists rather than allowing it to be concentrated on one (laying T&G flooring across the thickened joists would have achived much the same result with less effort).
For storing junk it should be fine... Although having said that see this:
Using the loft as a "habitable space" however is a different matter. Stored items tend to stay put and not move about much (one hopes!) People on the other do. This poses much more stringent requirements on a floor than one just used for storage.
You are unlikely to cause major structural damage if you don't alter the structure of the roof or any of the ties etc. The danger you face is that you will cause damage or sagging to the ceilings (although the saggging you see in my picture above is not exceptional - and is not visible from the room below as you eyes would need to be at ceiling level to see the deviation).
To use the loft as an extra room you need to think about not only the structural aspects but also the safety ones. What happens in the event of fire etc? Could the occupants of the space get out quickly if required? These are the sort of issues that a BCO will be looking at in addition to the stuctural ones.