Options
1 - ignore it. You can live with a wasp nest in your loft, though you might not want to
2 - Look for a new nest being built. If it's small enough then wait until it's a cold morning so the wasp/s are in the nest and remove it by hand. That's not literally by hand but a jar or something over the nest and card behind it and you can rmove the entire thing.
or if the nest is too big and you really do need to remove it
3 - Wait until it's cold so most of the wasps will be in the nest and then spray from as safe a position as you can with a wasp nest destroyer, that's usually permethrin in a foam.
or, if you can't see the nest but think you know where it is then scatter permethrin powder (usually sold as ant powder) on any of the routes you can see wasps getting in. As the wasps go in they should take the permethrin into the nest for you.
Whatever you choose, the basic rules are;
If you don't need to do anything then don't, wasps are useful
If you do need to do anything do it when it's cold so the wasps are in the nest
If you do need to do anything do it early in the year so the nest is as small as possible
I generally check my loft for nascent nests about now and deal with them while they're gold ball sized and have perhaps a dozen or so wasps at most, or even just a queen and eggs. When I moved to this house there was a nest in the loft about half a metre wide! That I just dealt with as best as possible but I couldn't finally get rid of it until the following winter.