@Travis_Potts There's a significant difference in hardware strength between the laptops you can get for under £400 and those around £450. I can't tell whether some of these laptops are in stock, but would any of them work?
https://www.argos.co.uk/product/1959318
https://www.box.co.uk/M509DA-EJ034T-ASUS-M509DA-EJ034T_2961552.html
https://www.argos.co.uk/product/9085538
(The three below are the same, except the last one has a larger hard drive.)
https://www.currys.co.uk/gbuk/computing/laptops/laptops/acer-aspire-3-a314-22-14-laptop-amd-ryzen-5-256-gb-ssd-black-10209211-pdt.html
https://www.argos.co.uk/product/8483935
https://www.laptopsdirect.co.uk/acer-aspire-3-a315-23-amd-ryzen-5-3500u-8gb-512gb-ssd-15.6-inch-fhd-windows-nx.hvuek.00h/version.asp
Other than the larger drive in the last one, they all have the same processor and integrated graphics chip, which should run the game on medium settings or perhaps medium-high with your combination of packs; plus 8 GB memory and enough storage for your needs. Some of them are lighter than others, but they're all under 2 kg.
Most of the cheaper laptops have at least one major flaw, usually either only 4 GB RAM or a 128 GB hard drive (or both). Some have a weaker processor and slower integrated graphics chip. While you should still be able to play on these, you'd get a hit in performance relative to the above. If you really want something £400 or under, these two are the same Acer model as the ones above, but with weaker processors; the first also has a 128 GB hard drive, which could be difficult but not impossible to manage:
https://www.currys.co.uk/gbuk/computing/laptops/laptops/acer-aspire-3-14-laptop-amd-ryzen-3-128-gb-ssd-silver-10209088-pdt.html
https://www.currys.co.uk/gbuk/computing/laptops/laptops/acer-aspire-3-15-6-laptop-intel-core-i3-1-tb-hdd-black-10208112-pdt.html
...Actually, I just saw a slightly older model laptop with an entry-level dedicated graphics card for under £400 as well. This one could give you the best performance in Sims 3 of all of these, that is if you're fine with the very small hard drive.
https://www.laptopsdirect.co.uk/acer-a315-55kg-core-i3-8130u-8gb-128gb-ssd-geforce-mx130-15.6-inch-windows-nx.hehek.002/version.asp
Whether a 128 GB drive is manageable or not depends on how much data you need to stora aside from Sims-related files. The base game and your packs will probably take up 25 GB; saves are small files, but custom content can take up a lot of space. You'd want 50 GB for Windows itself plus enough room for it to maneuver. Hard drives are never as large as they claim, so you could probably only count on about 115 GB total, which leaves 40 GB for everything else. If you're not sure whether that's enough, take a look at your current hard drive to see how much storage you're using now.
If you have more questions, please feel free to ask.