@camillamayday Sorry for the late reply. While £700 isn't a lot by the standards of gaming PCs, you can definitely something that can run Sims 4, all current packs included, on ultra graphics settings. Mods shouldn't make much of a difference, although high-resolution custom content can, especially on a system that doesn't have a lot of extra room. You should still be fine; I'm just saying don't expect a £700 machine to be a beast that can handle whatever you throw at it.
Anyway, these are the best options I found within your budget:
https://www.currys.co.uk/gbuk/gaming/pc-gaming/gaming-pcs/acer-nitro-n50-110-gaming-pc-ryzen-5-gtx-1650-1-tb-hdd-10209356-pdt.html
https://www.cyberpowersystem.co.uk/system/ultra-33-gtx-next-day-pc
https://www.amazon.co.uk/CyberpowerPC-Wyvern-580-Gaming-PC/dp/B079YZRZP9
https://www.amazon.co.uk/CyberpowerPC-Wyvern-1650-Gaming-PC/dp/B07D4LZNQ2
The second one has the weakest processor of the group, but it also has a solid state drive, which, while not strictly necessary, will make loading times noticeably faster. It's also a quality of life issue: Windows will run faster on an SSD, as will many applications. You could add an SSD to the others, or at least definitely the third and fourth, although you'd want to reinstall Windows on it. (That's a bit of a project but not difficult at all.)
The third computer also has 16 GB of RAM, which while also not necessary for Sims 4 will make heavy multitasking easier.
The first three computers have a motherboard that would take a processor upgrade up to and including the strongest consumer-grade AMD processors available. I'm sure you don't want a £700 CPU at this point, but the mid-tier option is around $200 U.S. when it's available, so that would be an easy upgrade if you ever needed it and assuming the chip shortage doesn't last forever. The Intel processor could also be upgraded, but the options are more limited; on the other hand, that CPU is better than the other options and should always be far beyond what Sims 4 needs.
The build quality of the Cyberpower models is higher than you'd get in the Acer, not that the Acer is bad: it's just that Cyberpower uses the same components you'd buy if you were building your own desktop, whereas companies like Acer, Dell, and HP often use cheaper proprietary components. On the other hand, many people would prefer to purchase from a company with physical stores, and if you're one of those people, the Acer from Currys is a good option.
If you have more questions, please feel free to ask.
