Can I ask why you 'need' a £300ish board? What is the need exactly? You'd also need to make sure that non of the SATA's are knocked out using 2x M.2 NVMe drives. If you know you are going to have to change the CPU to get Gen 4 PCI-E, then why bother spending big money on a board that might actually never get it? Intel have not committed 100% to the next CPU's being on LGA1200 having it, so you are risking a lot on a 'maybe'.
Until you have your hands on your new 3000 series cards DO NOT BUY any other parts, that is just a fools game. You'll end up regretting it if you get stuck with a 2 month wait for a card, and a 10900K, motherboard and RAM that may get outclassed by something at 30-50% less cost, and you can't make the most of it since you are on a wait list for a GPU.
Until you have your hands on your new 3000 series cards DO NOT BUY any other parts, that is just a fools game. You'll end up regretting it if you get stuck with a 2 month wait for a card, and a 10900K, motherboard and RAM that may get outclassed by something at 30-50% less cost, and you can't make the most of it since you are on a wait list for a GPU.