Does such a thing exist? Looking in UK, can’t find a pound sign on my ipad keyboard though lol. With MC I’d like to be able to use resource packs and run shaders. With sims 3 I’d like at least medium settings with most things and high would be good. I’d also be open to buying secondhand if it’s worth it and anyone can recommend what to look for?
For running Minecraft and the Sims 3 yes but shaders not so much as they are intense on performance. Even my PC which is fairly average can't run them that well, mostly higher end gaming PC (which usually aren't for that price but the older ones could) can run Shaders well. I can easily play Minecraft with many mods which also requires some good specs but just not Shaders.
I don't know how much 400 pounds is compared to $AUD but I assume it's possible, I got mine for $800 AUD in 2016 and it's an average PC that has worked out for what I use it for, I assume many in the England/UK cost less and many for average/entry level specs are about that now, but I can only guess. I'd say look at Minecraft and Sims 3's specifications and try to find PC's around that on Amazon, major stores online prices or local technology stores in your area.
Sims 3 seems to easily playable but Minecraft needs a bit more than it used to and as your using it for resource packs and the base game it is very much playable, just not with Shaders that well.
Minecraft's specs: https://help.minecraft.net/hc/en-us/articles/360035131371-Minecraft-Java-Edition-system-requirements- I'd say I probably have some parts that are similar to the recommended so if you can find one with close to those you'll be fine to play both of them. If you want an idea of what shaders are like with my specs look to this review I did on a Modpack with some and look at the FPS counter, the memory one is kind of important but if FPS is more what you want to consider for it being playable then look at that (though I have a few video settings tweaked from the default):