I think the reason is that UK is a very strong market for PS4, and at the same time a relatively weak one for Switch. It's not that PS4 is not going to be as strong in other markets: rather, it's that Switch is going to stronger than they are in the UK (and that XB1 is weaker outside of UK).You think the PS4 share will decarease more than Xbox worldwide? If anything it should increase with Xbox having a much bigger drop.
In the end, it's a rough estimate that will also hinge on how strong Switch version will be in the US. Benji said that it looks like PS4 > XB1 + Switch, but if that's something like 55/35/20, then that will build towards my split. If Europe (outside of UK) can do a 60/30/10 for Switch, then we should arrive at my estimate, maybe somewhat more towards 65/23/12 or so. I can't really predict this to the percentage accurate, but I do think Switch will do a bit better percentage-wise, and will steal single digit percentage share compared to UK. PS4 is still by far the biggest seller, of course.
You might be right that I am pegging the XB1 share too high, though, considering even UK (second strongest market worldwide) did only 18%. Maybe a 65/25/10 makes more sense?