OP, you sound like me
I live in a small flat too
I've also had many experiences of chucking things out and then wishing I hadn't as I tend to get rid of too much in a desperate attempt to reclaim space!
the first thing I will say is, if you normally lose weight in summer, don't chuck out the smaller clothes. (I am guessing you are not filthy rich?)
I agree about keeping paperwork - you could scan things in or take photos on iphone etc and make sure they are backed up but I like being able to put my hands on paperwork - just today I had a "did I open a new ISA this year or do a transfer" moment! Much easier to look at paperwork than scanned documents, in my opinion.
I have box files, which don't have a big foot print, so to speak, and hold a lot. They are not pretty but they don't need to be. You can get pretty ones from Amazon, Wilko, WHSmith if you feel the need.
oh and I have a lovely file box that someone gave me from Karenza but I think that's quite pricey for one box? Nice to have as a gift though!
www.karenzaandco.com/magnetic-closure-a4-floral-storage-box/
If you have anywhere that can fit extra storage, under bed or on top of wardrobe (e.g. in a big box) use that, because it might mean you can keep stuff you would otherwise get rid of. Again, not pretty stuff but the plastic things with wheels that store things under the bed are handy. Re boxes I have a couple of local shops who sell big storage boxes in neutral colours but if you don't, try Argos or something.
Re cleaning, it's easy to have 100 cleaning items I find, so I have decluttered and pretty much use water/washing up liquid diluted to various things - obviously I have loo bleach still.
I'd start with wherever you think you will free up most space. Don't attempt to do more than that room in a day I'd say.
I also have a massive picnic hamper type thing - admittedly it was given to me - but great for kitchen storage - I have an open plan kitchen/lounge space and both are tiny, so these are good options for hiding away kitchen towel, loo roll, drink cans.
I also put colours together in wardrobe, so if you have 100 black tshirts and you can spare 50, you know you can get rid of them. You could do the same with DD clothes - also prevents that moment of seeing something on sale and thinking "should I buy that?" - it's easier to remember what you have if it is in colour blocks.