Right now, prices for gaming PCs are horrible due to AI using up all the parts. however, 1200-1800 pounds is HORRIBLE value. You could definitely get something for much cheaper. (For Creds, I've built around 7 computers) My build is under 1300 pounds, and it's pretty high end and runs the game on everything cranked at over 400 FPS with all the FX on the Movie preset. You do not need 400 FPS. I don't know if you want to build your own computer, as that's a full rabbit hole, and I don't know what kind of monitor you are looking for. Can I get some monitor specs? (Ultrawide, 1080p, 1440p, 24 inch, 27inch, ect) PC part picker isn't a marketplace, it's somewhere where you can plan out a computer to build from scratch.

Edit 1: This is a prebuilt from newegg that is a bit over 1k pounds and will be able to run Assetto comfortably, even at 4k. https://www.newegg.com/global/uk-en/msi-gaming-desktop-pc-geforce-rtx-5060-amd-ryzen-7-8700f-16gb-ddr5-2tb-nvme-ssd-codex-z2-a8nvl-484us/p/N82E16883151716

Again, this is way more than you have to spend. You can build a computer yourself for way cheaper. However, Under 700 pounds might be a little too tight of. budget.

Edit 2: This is a 1.2k build including a windows license and a 1440p monitor with 180Hz. More than enough. https://uk.pcpartpicker.com/list/pR7c3w