Does it have to be carbon? Plenty of ally bikes out there that will give maybe 1lb or so away in the frame weight, but leave £500 or more in your wallet to spend on other things…
For instance, at £1099 the Grand Canyon AL SLX 7.9 is only a touch heavier but £550 cheaper than its equivalent carbon version. Then there’s the alloy Cannondale F29’s on Paul’s Cycles at under £1k (were £2k). I’d probably say the money saved spent elsewhere (lighter wheels, tyres etc.) will make a bigger difference than the frame material will.
That said, there are some great carbon bikes out there for relatively little money too, and not all carbon is created equal… I’ve come across carbon “xc race” frames weighing over 1600g before (when an equivalent ally is no heavier), but then there’s some gems like the KTM Aera which is only about 1200g when even the lightest most expensive carbon MTB frames in the world are only just dipping below 1kg.
The weight is in the wheels which being non QR, can’t be swapped for my AC race wheels…
The weight on ANY “budget” XC bikes is always going to be in the wheels, and if you’re buying new, you’re not likely to be able to swap for your existing QR wheels as everything new is coming bolt thru pretty much.