Desktops are typically better for the money for several reasons: their hardware is stronger, their cooling is better, they're easier to repair and so they typically last longer. Of course, laptops are much more convenient. And for £800, you can get a laptop that should run Sims 3 on ultra graphics settings, even with all packs installed.
There's no large difference in performance in your price range—laptops with an Nvidia 1650 start below £700, and models with the next card up (a 1660 ti) are at least £900. You don't usually see 16 GB memory in laptops in this category, although you could add more on your own easily enough; while 256 GB storage is enough for Sims 3, it's better to get a larger hard drive so you don't have to worry about running out of space. That narrows things down considerably.
The only real questions left are what processor you want, if you have an opinion, and whether you're willing to spend a little extra, not a lot, to get a more highly rated model. If you just want the cheapest good option, the Acer I linked earlier, for £650, would be fine. The HP (£700) is more highly rated as a gaming laptop though:
https://www.box.co.uk/1L6F6EAABU-HP-Pavilion-Gaming-15-ec17007na_2958258.html
An alternative is this Asus, also £700.
https://www.argos.co.uk/product/9199527
These two have the same specs except for the stronger processor in the HP, although Sims 3 would run fine on either one.
An alternative is this Asus, for £750, which has the same specs as the above models except for an Intel CPU. I don't think this processor is better than the one in the HP, although some people prefer Intel models.
https://www.currys.co.uk/gbuk/computing/laptops/laptops/asus-tuf-fx505gt-15-6-gaming-laptop-intel-core-i5-gtx-1650-512-gb-ssd-10208029-pdt.html
The Asus A15 is highly rated, and it's also £750 here, with the same specs as the HP:
https://www.overclockers.co.uk/asus-tuf-gaming-a15-fa506ih-nvidia-gtx-1650-8gb-15.6-144hz-ryzen-r5-4600h-gaming-laptop-lt-2bf-as.html
Since I don't know exactly what details you're looking for, it's hard to say what the best choice would be for you. If these aren't quite what you want, let me know what you'd like to see, and I'll have a look later today.
