Thanks
@Vince, damn shame about the motherboard.
If you've a fairly strict budget of £500 this would be functional:
My basket at OcUK:
Total: £476.96 (includes delivery: £7.99)
I'm listing the above purely due to current stock levels on OCUK, there's not a lot of AM4 options, although I believe the 12400F edges out the 5600/5700 in games anyway and X3D options are ridiculously expensive assuming you can even find one. It's much faster than your old 2600 regardless, so you will at least see good uplift in whatever tasks you do including gaming.
The motherboard is a bit light weight but should be fine for a 12400F from checking some reviews, the biggest downside is that it only has two RAM slots, meaning if you wanted/needed to upgrade to 32gb you would need to source a 2x16gb set, although selling your old RAM would offset this.
The Intel B580 should be at least double the performance of the 1660, and of course there's the added benefit of the extra RAM and RT support for the odd game that requires it like the latest Doom.
Honestly, you'd probably be better off looking for a second hand AM4/LG1700 bundle, with some luck you'll save enough to push into 9060XT 16gb territory, and/or hopefully a bump in RAM to 32gb.