Select the corner UVs marked with white circles, and pin them: UVs > Pin § Then move them to the pink crosses so they align in a rectangle shape. Now you can do another LSCM, and it calculates it all again, but keeping the UVs that you pinned in the same spot. It should turn out much more like a nice rectangular grid, which you can then clean up by aligning rows and columns of UVs.
Hmm, yes sometimes it’s done that for me too You may be able to get it a bit neater by pinning that top middle UV and moving it down a little. You can either try manually cleaning up the rows and columns with Weld/Align, or just try a cylindrical unwrap, like bm suggested.