What I did was buy the loft board from B&Q and lay these on top of the existing joists. There was insulation between the joists. then what I also did was buy the insulation rolls that come up in lidl or aldi every so often. They look like silver foil bubble wrap. I stapled these to the rafters to further insulate the area (and protect it from spiders - ref. wife). Making sure to provide adequate ventilation etc. In my case I was flooring a total small extension attic, so it was feasible. If you were flooring a small part of a larger attic, it might be an idea to create plastic "walls" to section off the floored part from the rest of the attic, to provide insulation and dust protection. heavy builders plastic or similar might be an idea. I think you can get magnetic closures that can run the length of the opening.
The idea (in my head), is to create a sort of "storage shed" in the attic.
And as Leo said, once there is easy access, e.g. a Stire attic stairs, you will find the amount of stuff that gets stored rises exponentially.