The DIY sheds will sell chipboard, which is basically rubbish, in small sheets, which are very expensive.
See what 18mm WBP ply costs, but it will be in large sheets so you will want a store that will cut it to size. B&Q might. Try first with a big piece of cardboard to see how big a sheet you can carry up. Probably you can get a piece 4ft x 2ft up the ladder and through the hatch; I can just manage a piece 8ft x 2 ft but quite hard work. The bigger the pieces, with the fewer cuts, the more rigid it will be. Measure the distance between the centres of the timbers, as you want all short joints to be on a timber. Screw them down. 50mm countersunk BZP screws will be suitable. Electrical cables must not be squashed under the boards or buried in insulation.
Due to the pitch of the roof you probably only need to board the central third of the loft, you won't be able to stand near the edges. The eaves must not be blocked to allow the ventilation that prevents condension. Holes in the ceilings especially for downlighters will be a problem.
Remember that modern trussed rafter roofs (with a W shaped structure) are carefully calculated to hold the weight of the tiles and some snow on top, and the weight of plaster ceilings below, and not to fall over in strong winds. It would be wasteful expense to make them as strong as the floor of a habitable room. So you can store your Xmas decs and pink pony collection up there, but not crates of books (which are very heavy), your piano, or groups of people. Screwed down boarding will add a little rigidity and spread the load slightly, but will not make the roof structure any stronger.
Older houses have cut roofs made up on site with timber sawn to size. They were not so precisely calculated to minimise material, and subject to width and timber sizes may be a bit stronger. But you need the measurements to be sure
Lofts are generally very dirty and if insulated with yellow fibre the dust may be irritant, so wear a dust mask with a plastic valve on the snout, and wear gloves. The timbers are rough sawn so will have splinters. There may be dead wildlife.