Is this possibly related to E10? E10 is the norm in the US but, as I understand it, E5 is more common in Europe. I don't know the chemistry but I do know that there are concerns about ethanol adversely affecting the life of engine seals, moreso in older vehicles not designed for it, but there's at least a fringe of people who insist it's an issue for modern engines as well.

I also know that there are some appreciable differences between EU and US standards for diesel (higher cetane number in Europe) which adversely impacts the performance of small diesel engines in the US, but I don't know if there are any similar issues with gasoline.

I'd also be curious about the manufacturer's specified oil. It is surprisingly common for new vehicles in the US to still specify conventional or blended oil, where 5k miles is a more common change interval. I've seen as high as 15k mile change interval specified in the manual for a US vehicle but the manual also specified full synthetic only (this was a turbocharged engine which tends to adversely impact life, I wonder if a similar naturally aspirated may have listed 20k).

In any case the "change oil every 3k miles" guideline is long gone in the US, with 5k-14k being more common, but nonetheless 3k was common for long enough that it seems to be thoroughly instilled into the minds of many members of the baby boomer generation. I think this has given a certain license to the chain lube shops, long a source of suspect business practices, to continue to use 3k miles for their window stickers. That said I've seen even Jiffy Lube put in a window sticker for 7k miles, for a blended oil. There's a story (I don't know if it's true) that the 3k mile recommendation stuck around for so long only because the domestic automakers were uninterested in going to the expense of longer testing, and that situation changed due to both media involvement (Consumer Reports for example took this on as a big issue in the '90s) and competition from the Japanese automakers that had invested more money into research.

5k actually seems unusually low for a new production vehicle and I would tend to think that must have been specified for conventional or at least blended. Outside of some older engines with detergent-related concerns pretty much the only thing keeping conventional oil on the market is price and more and more vehicles are specifying synthetic only.

Finally, temperatures across the UK are much more moderate than across the US. While synthetic oils have less issues this way, oil behavior in terms of actual viscosity and lubricity tends to vary appreciably by temperature, and worst of all mostly when it matters most right after startup. This has an adverse impact on both oil and engine life. It's become less common for US vehicles to specify different oil grades for different seasons/temperature ranges (although some still do), but US maintenance guidelines are still going to be developed for a pretty wide temperature range that notably includes ambient temperatures in the 80-100F/26-37C range that significantly thin oil and are uncommon in the UK. A similar problem exists at the low end, in parts of the US cold start temperatures of 20F/-7C are routine and most vehicle manuals no longer call for block heat until you get down to nearly 0F. Oil can take a long time to "limber" starting from these low temperatures and that's hard on engine life and puts a lot of metal shavings in the oil. Manufacturers kind of have to set their maintenance recommendations at "lowest common denominator" since practical experience with seasonal/climate-based oil recommendations have shown that vehicle owners and maintenance shops are both extremely bad at following them.