Make the most of sunny autumn days. Place a tile, bricks or small paving slab under squash to reflect heat and warmth up; if all else fails, a piece of black weedproof fabric will do.
Keep a very close eye on the weather forecasts. Once there is a danger of frost, squash should be harvested. Take them in somewhhere frost free, but don't store them yet.
American gardeners are expert squash growers and they understand the importance of ripening ('curing', they call it) the squash after they have been cut. Lay your squash in a warm sunny place; in a dry greenhouse is good if you have space, or outside on a bench or table. Take them inside if frost is forecast. Give them as long in the sun as you can, till the skins go hard. This is a good sign.
Once the skins have hardened up, the flavour will be fully developed and the squash will keep better. Butternut are usually OK till Christmas if fully ripened - check your squash regularly in storage and use the least ripe first! The first sign of deterioration is usually seen where the stalk joins the squash.
If you want a squash which will keep right round till spring, try one of the blue-grey ones like widely-available Crown Prince, or the rarer Queensland Blue. In most seasons these do better than butternut anyway.