No, if it has recently been reroofed and is now felted, it will not get much new dirt in it.
The dirt is probably black, and is 80 years of soot and dirt which settled on the roof and blew in through the gaps between the old tiles. The new felt (or hopefully breathable membrane) will prevent more dirt getting in that way, and I bet there is hardly any soot or factory pollution in your area now.
Buy a canister vacuum, with a replaceable cartridge filter (and buy a spare) and the huge paper bags that go inside it (this will reduce the clogging of the cartridge) and give the loft a really good clean. Wear a dust mak and goggles as well as gloves.
If it has old yellow fibreglass insulation, I would personally take it up, bag it, and take it to the tip before cleaning. It sheds irritating dust and fibres, and your is probably full of the black dirt. You can now get brown mineral wool treated with Ecose (look at the wrapper) which is a finer, softer filament, and the Ecose prevents it shedding dust and fibres. I will never use yellow fibreglass again.
Loft insulation is not expensive, and you might even get a subsidy if you have it done at the same time as cavity fill.
The black dirt is very heavy, like sand, so don't try to carry it through the house in bin bags or they may burst. Anything you carry down from the loft, clean and bag up before taking it down from the loft.
An electrician can very easily add loft lighting for you, because the lighting circuit for the upstairs lights is in the loft.
Fit the stiff plastic foam lagging round any waterpipes up there, and tank jackets round any water tanks.
You can lay loft flooring after you have cleaned it.