Now is the time for friends. We managed to declutter and prep for emigration in about 2 weeks flat including completely redecorating and folding up a business. The only reason it was possible was my wonderful friends. I put everything I didn't need in one place. They sorted into eBay, charity shop, and tip; put things up for sale or took them where they needed to go. When the eBay stuff sold they even packaged it up and addressed it so that I only had to take it all to the post office. My advice, your friends are, well, your friends! Good luck.
Don't use binbags. Something valuable is bound to be thrown out by mistake. You can get huge clear plastic bags from the same shelf of the supermart, they are sold as Recycling bags. You can see what is in them, which is a great advantage.
I think it's fine unless you are moving to a much smaller house. You won't have the pressure of running out of time if you declutter after the move, but sometimes that means you won't get around to it.
I did a major declutter for over a year, about 3 years before we moved house. But my ds outgrew lots of stuff after that and I continued to get rid of stuff when I could and I still do. When it came to moving, I mostly just packed. There is no way I could have done a major declutter and packed and moved. But I let the removers get rid of some furniture which was only fit for wood recycling and landfill. Slightly more expensive than using a waste company or the council but much easier.
BTW you don't need a spare/empty room to put packed boxes in if you're packing. I just packed stuff and had boxes in each room.
Personally, I like to do most of the clearing out once I'm in the new house. It helps a lot because you can actually see how much storage space you have, and you're actually putting stuff in it's place, so you can see if it's useful or not anymore. Otherwise, I tend to imagine it while I'm packing, and think, "Yes, that might fit there..." Or I'll find my wardrobe isn't actually as big as I imagined [sob] so I'll need to be ruthless with some clothes. [off to mourn the loss of clothes]
We did both - we thought we'd de cluttered to sell, then we got the removal firm to pack for us.
Once we were living our new life we found that we hasn't unpacked some boxes, so on inspection they went to charity, and when we transferred things into smaller boxes for the loft, we chucked a load away then too.
Decluttering is a lifelong process, not a one off job!
I guess part of my reticence to do it myself too is that we have now been waiting 5 months since we accepted an offer on our house and a couple of weeks more than that since our offer was accepted... I guess I am finding it hard to believe this move will ever actually happen!
I'm doing the opposite and all my friends who've used packers think I'm crazy but it's what works for us. It does also mean living in maze of boxes which isn't easy especially if you have no spare space, believe me we live in a big house and everywhere I look seems to be brown cardboard ATM! You need to do what works for you, don't worry about what others think. Moving is stressful enough without that.
We should exchange any day now (although I've been saying that for about two months!) and we did initially make an effort to declutter / tidy but we really haven't had much time for a while with so many other things going on. We are getting packers in as we just don't have a spare room to box up and store stuff in so basically now I'm expecting the packers to pack everything up and then I will sort / declutter the other end once we move. Seems contrary to what most people do but I just don't feel like we have the time or space to do it before moving. Has anyone else done this?