Yes, and this is the thing the site is most insistent about: insulation is the same conversation in both directions.
Everything sold as a winter measure is also a summer measure, because it slows heat moving through the fabric of the house whichever way it is trying to go. A loft with a warm roof over it in August is losing that heat downwards into the bedroom, and insulation between you and it is the thing that stops that.
The one thing to be careful about is ventilation. Insulating without thinking about how the roof space breathes is how people end up with a condensation problem in winter that they did not have before, and that is a job worth getting right rather than fast.
Nothing on the site quotes a payback figure for this, deliberately, because it depends on what is there now and what you pay for heat. If anybody has done it and watched what the bedroom did the following summer, that is the number I would want.