Given how you've described your goals here—get the cheapest good option, play on ultra settings now, futureproof for a Sims 5 that may or may not ever exist—your best bet might be to get a custom build. You could get an Nvidia 1650, a graphics card that's relatively cheap but more than good enough for Sims 3, but select other components that would support a better card if and when you want to upgrade. GPUs are improving so rapidly these days, it's almost impossible to futureproof for games that haven't even been announced. It might be that in a few years, you could get a new card for £300 that's better than almost anything currently on the market besides the 2080 ti.
As far as Turing technology goes, there's no way Sims 3 can benefit, as igazor mentioned. Raytracing is amazing, but there are only a handful of games currently released that can even use it. So unless you're planning to play Control, or the new Wolfenstein, or some other new game that can take advantage of an RTX card, you might want to wait until you have another reason to want a stronger GPU.
On that note, you can absolutely stay under £1000 and get an excellent computer. As an example, this particular configuration, on
- CyberpowerPC's UK site, costs £980, and has:
- an AMD Ryzen 3600 processor
- Corsair liquid cooling for the CPU
- an Nvidia 1660 graphics card (swapping it out for a 1650 would save you £70 including VAT)
- 16 GB RAM 3200 MHz (AMD processors love fast RAM)
- a 512 GB Adata NVMe SSD (faster than a SATA SSD) plus a 1 TB HDD
- an MSI X470 motherboard, with room for multiple upgrades
- a Corsair 650W 80+ Bronze PSU
These are just the parts I picked out while poking around on the site for a bit, and trying to keep the price down without compromising on quality. It's very easy to swap out any particular part, other than the fact that some of the cheaper boards don't support certain hardware.
http://www.cyberpowersystem.co.uk/saved/1789745
There are other sites that do custom builds as well; I'm just more familiar with this one, and it gives more flexibility than some others.
If you'd rather have a prebuilt system, you can certainly still get decent hardware. The difference though is that it might be more difficult to upgrade later, or at the very least you might need to buy a new power supply. It just depends on the company that makes the PC and what components they choose to use. I will say though that you can do better than Currys. Those prices look a bit more expensive than is necessary. If you have particular components in mind, e.g. a 2060 or particular processor, I can try to find something a bit cheaper.