@SophieBxnks The reason I like to suggest custom builds, if people can afford them, is that you can get better quality parts than you would in a prebuilt model. Acer is a reputable company and wouldn't sell you a computer with bad hardware. But it's going to include the minimum reasonable components to support the processor and graphics card already included. It wouldn't make sense for any company to pay more to include parts that the current build doesn't need, even if those parts would be nice to have for future upgrades.
In practical terms, that means you'd be getting a lower-end motherboard, power supply, and maybe CPU cooler. Those parts would be fine for now, but they might not support an upgrade in a few years. The advantage of the build I linked (and others like it) is that all the included components will support an upgrade in the future. What it really means is that your son could decide, for example, he wants a new graphics card in a couple of years and merely swap out the old one without worrying about whether the rest of the computer can handle it. The mainboard in particular also supports PCIe 4.0, an upgrade that current hardware can't take advantage of, but that will probably become standard in a few years, at least for the high-end components.
Having said that, as long as it's properly maintained, the Acer should be able to run Sims 4 on ultra settings for the entire run of the game, no matter how many mods are thrown at it. With an Nvidia 2060 and an Intel i7 that's almost certainly of comparable quality, your son would never have an issue. The nice thing about being willing to spend £1,500 is that as long as you do a little bit of homework, you're going to get what you need and then some.
Edit: This is a slightly better configuration, at £1,068 including VAT:
http://www.cyberpowersystem.co.uk/saved/1857828
It's worth mentioning that a (very unnecessary for Sims 4) upgrade to an even stronger processor, the AMD 3700X, is £100 without VAT. Also in the category of unnecessary, there are a number of cases in various loud colors that your son might like, at various prices. You could in theory get a stronger graphics card as well, but there isn't much of a point in spending that money now—it would be more efficient to buy a much faster card in 2 or 3 years if your son eventually decides he has a use for it.
