What's the lightest you've backpacked,including tent,sleeping bag,cooking gear,change of clothing,wash kit,repair kit and emergency rations.I can't get lower than 15 kg.
Honestly don't see this as a problem for trips, especially this time of year where there is a lot of extra winter weight due to clothing.
Three to four days food may be nearly 2kg, carried water may be 0.5-1kg at the low end. I probably carry 5kg of clothing, maps and camp shoes if I'm doing long distance this time of year, including the pack. Some of the expedition size backpacks have more robust carry systems which push them in to the 1.5-2kg size alone. A sleeping system of mid to high quality could be shy of 3.5-4kg for a mat, tent, stakes, bag and drysacks. It'll cost you probably another £500 to drop this to 2kg, or 25p per gram.
Being realistic, you are on a budget and you want the most bang for your buck. You can't buy a new rucksack until you downsize everything else, so stick with the rucksack, and forget about buying new for the time being. Look on places like ebay, pick up some bargains, and first thing you want to focus on is the big four (sleeping bag, tent, rucksack and mat) - if you are happy with those, look at the 'comforts'. Do you really need these? Would you rather have less 'comforts' but be more comfortable by carrying less?
I'd suggest using hiking poles if you don't already for heavy loads, and set a budget for how much you want to spend to change around your gear. You don't need the newest, shiniest kit, but some improvements make a big difference. Especially in the 'big four' listed above. You said you have a new sleeping bag so forget about improving that, I'd personally look at a second hand lightweight tent and expect to pay something in the region of £200-300, as well as cutting the gear weight of what I'm carrying.
Overall you have two choices, the expensive way or the cheap way. The expensive way is to replace some of the items you are taking, with lighter alternatives. The cheap way is removing parts of your setup and realising you can do with less.
Lastly, don't be dazzled by ridiculously low pack weights and think you are Atlas by comparison. There are many UL hikers who carry absolutely minimum emergency kit, a shockingly inadequate first aid kit for anything more than a small cut or blisters, no bothy bags nor ultralight emergency bivvies. If they get in to trouble they are entirely reliant on walking off the mountain, or sending a friend for help.