We cut down one from our land every year for Christmas. This year I want to get it a bit earlier than usual (like this weekend), but we're not sure if it will be dead and horrible before Christmas or not. How long before Christmas can I cut it that it will still be alive and not falling apart on Christmas Eve (when we celebrate Christmas).
Don't do it. Norway's hold their needles poorly. From Top 10 Choices for Christmas Trees, where Norway spruce's were the 10th choice out of 10:
The Norway Spruce is a beautiful tree but does not hold its needles well, and should be purchased just just a week or so before December 25th. The National Christmas Tree Association notes that the "overall color of Norway spruce is fair to excellent, but needle retention is considered poor unless the trees are cut fresh and kept properly watered."
If your inside temperature is low and/or humidity is high, that will slow down needle drop. Since you're in the UK, I'm guessing that you have relatively warm winters (compared to New England) and high outside humidity - both of which increase the inside humidity.
When you buy a tree from a parking lot in the city, it was probably cut in November. I see the trucks driving by carrying them weeks and weeks before Christmas. But they are kept outside in the cold. If you want to decorate a tree starting Dec 1st, use an artificial one. If you want to use a natural tree, have it up for two weeks, tops.
My tradition is to put up the tree Dec 23rd or 24th. (When I was a child, the tree went up after the littlest had gone to bed Christmas Eve, and for my children I have always saved certain decorating for after they're asleep even though the tree is up.) Then we keep it up for "the twelve days" and sometimes need another day or two after that to get it all taken down. We cut from our property and some might well be Norway Spruce from a quick glance at pictures of them. we keep it in a stand that holds 2+ litres of water, and keep it topped up. By the last day it is dropping needles, but it's not awful. It's possible that it's dropping needles because we've become complacent about watering it, and that you could get a longer life by being more conscientious - my answer is as much about human nature as trees in that case.
You will gain lifetime by cutting it fresh. But I don't think I could push it to four or five weeks.
In Italy those trees go to die in max 2 weeks. Lately we decided to buy them with roots. After Christmas we go to plant them in mountain, in our terrain or not.
Make a hole deep enough, cover the roots and water it. We found them every time alive in Italy's mountains.